Replace vim-gitgutter with vim-signify

vim-gitgutter is buggy. vim-signify might be better, it does allow
configuration of which events trigger a sign column update and this
patch takes advantage of that. Firstly, the `CursorHold` and
`CursorHoldI` events are disabled and replaced with `InsertLeave` and
`TextChanged` events. This makes the sign column updates immediate, and
since vim-signify is asynchronous it's faster too.

plugin/autocmds.vim

@@ -25,4 +25,7 @@ augroup benieAugroup
   au BufNewFile * silent! call snippet#template()
   " Do the same when filetype changes to help
   au FileType help silent! call snippet#template()
+
+  " Augment vim-signify by modifying it's autocmds
+  au User SignifyAutocmds call do#signify()
 augroup END