Add support for linting git commit message files (#1233)

doc/ale-gitcommit.txt

@@ -0,0 +1,42 @@
+===============================================================================
+ALE Git Commit Integration                              *ale-gitcommit-options*
+
+
+===============================================================================
+gitlint                                                 *ale-gitcommit-gitlint*
+
+g:ale_gitcommit_gitlint_executable         *g:ale_gitcommit_gitlint_executable*
+                                           *b:ale_gitcommit_gitlint_executable*
+  Type: |String|
+  Default: `'gitlint'`
+
+  This variable can be changed to modify the executable used for gitlint.
+
+
+g:ale_gitcommit_gitlint_options               *g:ale_gitcommit_gitlint_options*
+                                              *b:ale_gitcommit_gitlint_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be changed to add command-line arguments to the gitlint
+  invocation.
+
+  For example, to dinamically set the gitlint configuration file path, you
+  may want to set >
+
+  let g:ale_gitcommit_gitlint_options = '-C /home/user/.config/gitlint.ini'
+<
+
+g:ale_gitcommit_gitlint_use_global         *g:ale_gitcommit_gitlint_use_global*
+                                           *b:ale_gitcommit_gitlint_use_global*
+  Type: |Number|
+  Default: `0`
+
+  This variable controls whether or not ALE will search for gitlint in a
+  virtualenv directory first. If this variable is set to `1`, then ALE will
+  always use |g:ale_gitcommit_gitlint_executable| for the executable path.
+
+  Both variables can be set with `b:` buffer variables instead.
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl: