Support 256-color terminal environments

Fixes #110

With this commit, delta enters "24 bit color mode" iff either of the
following are true:

1. The --24-bit-color command line option value is "always"

2. The --24-bit-color command line option value is "auto" and the
   COLORTERM environment variable is set to "truecolor" or "24bit".
   See
   https://gist.github.com/XVilka/8346728#true-color-detection
   https://github.com/sharkdp/bat/blob/7779d9f6221b3e98c43a43ceb5596ba285fdf4f8/src/bin/bat/app.rs#L29-L33

Otherwise, delta enters "8-bit color mode".

In "24 bit color mode", delta will

1. Emit 24-bit RGB color shell escape sequences that will only be
   displayed correctly by a terminal application that supports 24 bit
   colors.

2. Select default background colors that will only be displayed
   correctly by a terminal application that supports 24 bit colors.

In "8-bit color mode", delta will

1. Emit color shell escape sequences specifying the entry in the ANSI
   256 color palette that is closest (according to the ansi_colours
   library) to the requested color.

2. Select default background colors that will work well in a terminal
   application that supports 8-bit color but not 24-bit color.

1 files changed, 28 insertions, 13 deletions

diff --git a/src/draw.rs b/src/draw.rs
index 751f99f9..03b93bdb 100644
--- a/src/draw.rs
+++ b/src/draw.rs
@@ -15,6 +15,7 @@ pub fn write_boxed(
     _line_width: usize, // ignored
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let up_left = if heavy {
         box_drawing::heavy::UP_LEFT
@@ -22,8 +23,12 @@ pub fn write_boxed(
         box_drawing::light::UP_LEFT
     };
     let box_width = strip_ansi_codes(text).graphemes(true).count() + 1;
-    write_boxed_partial(writer, text, box_width, color, heavy)?;
-    write!(writer, "{}", paint::paint_text_foreground(up_left, color))?;
+    write_boxed_partial(writer, text, box_width, color, heavy, true_color)?;
+    write!(
+        writer,
+        "{}",
+        paint::paint_text_foreground(up_left, color, true_color)
+    )?;
     Ok(())
 }
 
@@ -35,9 +40,10 @@ pub fn write_boxed_with_line(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let box_width = strip_ansi_codes(text).graphemes(true).count() + 1;
-    write_boxed_with_horizontal_whisker(writer, text, box_width, color, heavy)?;
+    write_boxed_with_horizontal_whisker(writer, text, box_width, color, heavy, true_color)?;
     write_horizontal_line(
         writer,
         if line_width > box_width {
@@ -47,6 +53,7 @@ pub fn write_boxed_with_line(
         },
         color,
         heavy,
+        true_color,
     )?;
     write!(writer, "\n")?;
     Ok(())
@@ -58,9 +65,14 @@ pub fn write_underlined(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
-    writeln!(writer, "{}", paint::paint_text_foreground(text, color))?;
-    write_horizontal_line(writer, line_width - 1, color, heavy)?;
+    writeln!(
+        writer,
+        "{}",
+        paint::paint_text_foreground(text, color, true_color)
+    )?;
+    write_horizontal_line(writer, line_width - 1, color, heavy, true_color)?;
     write!(writer, "\n")?;
     Ok(())
 }
@@ -70,6 +82,7 @@ fn write_horizontal_line(
     line_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let horizontal = if heavy {
         box_drawing::heavy::HORIZONTAL
@@ -79,7 +92,7 @@ fn write_horizontal_line(
     write!(
         writer,
         "{}",
-        paint::paint_text_foreground(&horizontal.repeat(line_width), color)
+        paint::paint_text_foreground(&horizontal.repeat(line_width), color, true_color)
     )
 }
 
@@ -89,17 +102,18 @@ pub fn write_boxed_with_horizontal_whisker(
     box_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let up_horizontal = if heavy {
         box_drawing::heavy::UP_HORIZONTAL
     } else {
         box_drawing::light::UP_HORIZONTAL
     };
-    write_boxed_partial(writer, text, box_width, color, heavy)?;
+    write_boxed_partial(writer, text, box_width, color, heavy, true_color)?;
     write!(
         writer,
         "{}",
-        paint::paint_text_foreground(up_horizontal, color)
+        paint::paint_text_foreground(up_horizontal, color, true_color)
     )?;
     Ok(())
 }
@@ -110,6 +124,7 @@ fn write_boxed_partial(
     box_width: usize,
     color: Color,
     heavy: bool,
+    true_color: bool,
 ) -> std::io::Result<()> {
     let horizontal = if heavy {
         box_drawing::heavy::HORIZONTAL
@@ -131,10 +146,10 @@ fn write_boxed_partial(
     write!(
         writer,
         "{}{}\n{} {}\n{}",
-        paint::paint_text_foreground(&horizontal_edge, color),
-        paint::paint_text_foreground(down_left, color),
-        paint::paint_text_foreground(text, color),
-        paint::paint_text_foreground(vertical, color),
-        paint::paint_text_foreground(&horizontal_edge, color),
+        paint::paint_text_foreground(&horizontal_edge, color, true_color),
+        paint::paint_text_foreground(down_left, color, true_color),
+        paint::paint_text_foreground(text, color, true_color),
+        paint::paint_text_foreground(vertical, color, true_color),
+        paint::paint_text_foreground(&horizontal_edge, color, true_color),
     )
 }