normalized_name: ignore case and prefix/suffix with underscore
Merge pull request #340 from blueyed/improve-normalized_name

Improve normalized_name: case, underscore, .vim suffix
Do not use `silent(!)` in `s:on_source`

This suppressed any messages during sourcing and executing
VimEnter/GUIEnter events.
Wrap `s:on_source` and its inner 'source' with try/catch