Improve load snippet ui

This is approximately what [I proposed in #417][1], but I had forgotten to think about saving, this patch does:

* `C-c C-l` - just load
* `C-c C-c` - load, save and bury
* `C-u C-c C-c` - load, save and kill

Furthermore, saving also works on existing snippets, not just new ones.

[1]: https://github.com/capitaomorte/yasnippet/issues/417#issuecomment-26965236