Implementation of 'bat --diff'

This adds a new `--diff` option that can be used to only show lines
close to Git changes (added/removed/modified lines). The amount of
additional context can be controlled with `--diff-context=N`.

closes #23

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 
 ## Features
+
+- Add a new `--diff` option that can be used to only show lines surrounding
+  Git changes, i.e. added, removed or modified lines. The amount of additional
+  context can be controlled with `--diff-context=N`. See #23 and #940
+
 ## Bugfixes
 ## Other
 ## `bat` as a library