Replace deprecated `error_chain` crate with `anyhow` (#1405)

The `error_chain` crate is now deprecated for a long time and `anyhow`
has proven to be a popular replacement for applications.

This also improves the current error messages for panics.

```
PAGER='"less' git show
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: \
Error(Msg("Could not parse pager command."), State { next_error: Some(ParseError), \
backtrace: InternalBacktrace })', src/main.rs:136:88
```

```
PAGER='"less' git show
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Could not parse pager command.

Caused by:
    missing closing quote', src/main.rs:125:88
```

1 files changed, 2 insertions, 3 deletions

diff --git a/src/utils/bat/output.rs b/src/utils/bat/output.rs
index de02b18e..ca062e1b 100644
--- a/src/utils/bat/output.rs
+++ b/src/utils/bat/output.rs
@@ -74,8 +74,7 @@ impl OutputType {
             .or(pager_from_env)
             .unwrap_or_else(|| String::from("less"));
 
-        let pagerflags =
-            shell_words::split(&pager).chain_err(|| "Could not parse pager command.")?;
+        let pagerflags = shell_words::split(&pager).context("Could not parse pager command.")?;
 
         Ok(match pagerflags.split_first() {
             Some((pager_name, args)) => {
@@ -117,7 +116,7 @@ impl OutputType {
             OutputType::Pager(ref mut command) => command
                 .stdin
                 .as_mut()
-                .chain_err(|| "Could not open stdin for pager")?,
+                .context("Could not open stdin for pager")?,
             OutputType::Stdout(ref mut handle) => handle,
         })
     }