Feat: Add Color Configuration (#82)

This MR adds the ability to customize colors for the discussion tree

after/syntax/gitlab.vim

@@ -0,0 +1,15 @@
+if filereadable($VIMRUNTIME . '/syntax/markdown.vim')
+  source $VIMRUNTIME/syntax/markdown.vim
+endif
+
+syntax match Username "@\w\+"
+syntax match Date "\v\d+\s+\w+\s+ago"
+syntax match ChevronDown ""
+syntax match ChevronRight ""
+
+highlight link Username GitlabUsername
+highlight link Date GitlabDate
+highlight link ChevronDown GitlabChevron
+highlight link ChevronRight GitlabChevron
+
+let b:current_syntax = "gitlab"