Only remove a snippet in insert mode

Part 2 of the solution to #159
Typo
Update mirrors only when in insert mode

Fixes #163
Use globpath() workaround on Windows
Discard snippets with empty triggers
Interpret backslash escapes correctly

This solution is a bit hacky. The actual parser on the nested branch is
much preferable.

Fixes #164
End a snippet right after entering the 0th tabstop
Merge remote-tracking branch 'origin/master'