Support snippet templates and add help snippets

Add `autocmd` to automatically expand the `_template` snippet on the
`BufNewFile` event, if the expansion fails because the `_template`
snippet does not exist for the current `filetype` undo the changes. Do
the same thing when the `filetype` changes to `help`, since these are
`text` files to begin with the first `autocmd` has no effect.

autoload/snippet.vim

@@ -0,0 +1,16 @@
+" Description: Expand snippet on file creation.
+
+" Attempt to expand the _template snippet if this is a new file.
+" https://noahfrederick.com/log/vim-templates-with-ultisnips-and-projectionist
+function! snippet#template() abort
+  " Return if non-empty buffer or file exists.
+  if !(line('$') == 1 && getline('$') ==# '') || filereadable(expand('%'))
+    return
+  endif
+  " Attempt to expand the _template snippet.
+  execute "normal! i_template\<C-r>=UltiSnips#ExpandSnippet()\<CR>"
+  if g:ulti_expand_res == 0
+    " Expansions failed, undo insert.
+    silent! undo
+  endif
+endfunction