Use hooks.goodcommit.color for color config if set

This change:

* Favour `hooks.goodcommit.color` for color config if set, fallback to
  `color.ui` if not set, default to `auto` if none set.
* Adds 'Configuration' section to README.

Author: tommarshall

README.md

@@ -65,6 +65,16 @@ n - abort commit
 ? - print help
 ```
 
+## Configuration
+
+The default colour setting is `auto`, but the hook will use `git`'s `color.ui` config setting if defined. You override the colour setting for the hook with:
+
+```
+git config --global hooks.goodcommit.color "never"
+```
+
+Supported values are `always`, `auto`, `never` and `false`.
+
 ## Dependencies
 
 None, other than Bash.

hook.sh

@@ -25,7 +25,7 @@ NC=
 #
 
 set_colors() {
-  local default_color=$(git config --get color.ui || echo 'auto')
+  local default_color=$(git config --get hooks.goodcommit.color || git config --get color.ui || echo 'auto')
   if [[ $default_color == 'always' ]] || [[ $default_color == 'auto' && -t 1 ]]; then
     RED='\033[1;31m'
     YELLOW='\033[1;33m'