Add vim-snippets to an optional-bundle area.
Upgrade UltiSnips to 3.0.
Stop clearing snippets and add priorities to our snippets.

`clearsnippets` no longer works as expected due to changes in UltiSnips,
and it seems likely it'll go away all together.  Also, adjust the
priority of our snippets so that user defined ones will have a greater
priority by default.
Update notes for UltiSnips.