Add noRepeat to relevent frontend commands

The use of Vomnibar commands with a numerical prefix > 1 is broken and
non-trivial to fix. This patch introduces the `noRepeat` property for
frontend commands, applying it to all Vomnibar commands and several
others.
Prompt for confirmation when executing large numbers of repeats

Implement `repeatLimit`, to prompt for confirmation before repeating a
command a large number of times, and apply it to tab creation/deletion
commands.
Merge branch 'limitedRepeats' of github.com:mrmr1993/vimium into mrmr1993-limitedRepeats
Limit repeats, minor refactoring.