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

test/handler/test_gitlint_handler.vader

@@ -0,0 +1,37 @@
+Before:
+    runtime ale_linters/gitcommit/gitlint.vim
+
+After:
+    call ale#linter#Reset()
+
+Execute(The gitlint handler should handle basic warnings and syntax errors):
+  AssertEqual
+  \ [
+  \   {
+  \     'lnum': 1,
+  \     'type': 'E',
+  \     'text': 'B6: Body message is missing',
+  \   },
+  \   {
+  \     'lnum': 2,
+  \     'type': 'E',
+  \     'text': 'B4: Second line is not empty: "to send to upstream"',
+  \   },
+  \   {
+  \     'lnum': 3,
+  \     'type': 'E',
+  \     'text': 'B5: Body message is too short (19<20): "to send to upstream"',
+  \   },
+  \   {
+  \     'lnum': 8,
+  \     'type': 'E',
+  \     'text': 'T1: Title exceeds max length (92>72): "some very long commit subject line where the author can''t wait to explain what he just fixed"'
+  \   },
+  \ ],
+  \ ale_linters#gitcommit#gitlint#Handle(1, [
+  \   '1: B6 Body message is missing',
+  \   '2: B4 Second line is not empty: "to send to upstream"',
+  \   '3: B5 Body message is too short (19<20): "to send to upstream"',
+  \   '8: T1 Title exceeds max length (92>72): "some very long commit subject line where the author can''t wait to explain what he just fixed"'
+  \ ])
+