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.

UltiSnips/help.snippets

@@ -0,0 +1,24 @@
+snippet _template "help file template"
+*`!p snip.rv = snip.fn`*	For Vim version 8.0	Last change: `!p
+from datetime import datetime
+snip.rv = datetime.now().strftime('%B %d, %Y')`
+
+$0
+
+ vim:tw=78:ts=8:ft=help:norl:
+endsnippet
+
+snippet s "help section"
+==============================================================================
+${1:1}. ${2:Section}`!p
+spaces = 78 - len(t[1]) - len(snip.basename) - (2 * len(t[2])) - 3
+snip.rv = spaces * ' ' + '*' + snip.basename + '-' + t[2].lower() + '*'`
+
+$0
+endsnippet
+
+snippet d "help detail"
+`!p spaces = 78 - len(t[1])
+snip.rv = spaces * ' '`*${1:}*
+$0
+endsnippet