Automatically add っ when typing double consonants

In doing this, the hiragana start point has to be reset to the first
roman character it finds in the converted string, rather than after the
end of it.
Add ん in the case of 'n' followed by a consonant
Activate vimes before motion-driven kanji conversion

Doing this means all the vimes keymappings, etc will be activated.
We have to disable the insert mode mapping on our way in, as we are
setting the value of g:kanji_start_point outside of insert mode and we
don't want it to be reset, however it needs to be re-enabled once we're
in, in case the user leaves insert mode without de-activating vimes.