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, 100 insertions, 8 deletions

diff --git a/src/style.rs b/src/style.rs
index ae483c8f..80256805 100644
--- a/src/style.rs
+++ b/src/style.rs
@@ -19,62 +19,154 @@ pub fn is_no_syntax_highlighting_theme_name(theme_name: &str) -> bool {
     theme_name.to_lowercase() == "none"
 }
 
-pub const LIGHT_THEME_MINUS_COLOR: Color = Color {
+pub fn get_minus_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_MINUS_COLOR,
+        (true, false) => LIGHT_THEME_MINUS_COLOR_256,
+        (false, true) => DARK_THEME_MINUS_COLOR,
+        (false, false) => DARK_THEME_MINUS_COLOR_256,
+    }
+}
+
+pub fn get_minus_emph_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_MINUS_EMPH_COLOR,
+        (true, false) => LIGHT_THEME_MINUS_EMPH_COLOR_256,
+        (false, true) => DARK_THEME_MINUS_EMPH_COLOR,
+        (false, false) => DARK_THEME_MINUS_EMPH_COLOR_256,
+    }
+}
+
+pub fn get_plus_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_PLUS_COLOR,
+        (true, false) => LIGHT_THEME_PLUS_COLOR_256,
+        (false, true) => DARK_THEME_PLUS_COLOR,
+        (false, false) => DARK_THEME_PLUS_COLOR_256,
+    }
+}
+
+pub fn get_plus_emph_color_default(is_light_mode: bool, is_true_color: bool) -> Color {
+    match (is_light_mode, is_true_color) {
+        (true, true) => LIGHT_THEME_PLUS_EMPH_COLOR,
+        (true, false) => LIGHT_THEME_PLUS_EMPH_COLOR_256,
+        (false, true) => DARK_THEME_PLUS_EMPH_COLOR,
+        (false, false) => DARK_THEME_PLUS_EMPH_COLOR_256,
+    }
+}
+
+const LIGHT_THEME_MINUS_COLOR: Color = Color {
     r: 0xff,
     g: 0xe0,
     b: 0xe0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_MINUS_EMPH_COLOR: Color = Color {
+const LIGHT_THEME_MINUS_COLOR_256: Color = Color {
+    r: 224,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_MINUS_EMPH_COLOR: Color = Color {
     r: 0xff,
     g: 0xc0,
     b: 0xc0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_PLUS_COLOR: Color = Color {
+const LIGHT_THEME_MINUS_EMPH_COLOR_256: Color = Color {
+    r: 217,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_PLUS_COLOR: Color = Color {
     r: 0xd0,
     g: 0xff,
     b: 0xd0,
     a: 0xff,
 };
 
-pub const LIGHT_THEME_PLUS_EMPH_COLOR: Color = Color {
+const LIGHT_THEME_PLUS_COLOR_256: Color = Color {
+    r: 194,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const LIGHT_THEME_PLUS_EMPH_COLOR: Color = Color {
     r: 0xa0,
     g: 0xef,
     b: 0xa0,
     a: 0xff,
 };
 
-pub const DARK_THEME_MINUS_COLOR: Color = Color {
+const LIGHT_THEME_PLUS_EMPH_COLOR_256: Color = Color {
+    r: 157,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_MINUS_COLOR: Color = Color {
     r: 0x3f,
     g: 0x00,
     b: 0x01,
     a: 0xff,
 };
 
-pub const DARK_THEME_MINUS_EMPH_COLOR: Color = Color {
+const DARK_THEME_MINUS_COLOR_256: Color = Color {
+    r: 52,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_MINUS_EMPH_COLOR: Color = Color {
     r: 0x90,
     g: 0x10,
     b: 0x11,
     a: 0xff,
 };
 
-pub const DARK_THEME_PLUS_COLOR: Color = Color {
+const DARK_THEME_MINUS_EMPH_COLOR_256: Color = Color {
+    r: 124,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_PLUS_COLOR: Color = Color {
     r: 0x00,
     g: 0x28,
     b: 0x00,
     a: 0xff,
 };
 
-pub const DARK_THEME_PLUS_EMPH_COLOR: Color = Color {
+const DARK_THEME_PLUS_COLOR_256: Color = Color {
+    r: 22,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
+const DARK_THEME_PLUS_EMPH_COLOR: Color = Color {
     r: 0x00,
     g: 0x60,
     b: 0x00,
     a: 0xff,
 };
 
+const DARK_THEME_PLUS_EMPH_COLOR_256: Color = Color {
+    r: 28,
+    g: 0x00,
+    b: 0x00,
+    a: 0x00,
+};
+
 /// A special color to specify that no color escape codes should be emitted.
 pub const NO_COLOR: Color = Color::BLACK;