Fix undo via `<Esc>:undojoin<cr>i` (applied via `s:get_left_motion`)

While this fixes the undo chaining in general, `<Left>` is still being
used in block-wise visual mode, where exiting insert mode would also
exit the block-wise mode.

Ref: https://github.com/Raimondi/delimitMate/issues/138
Return '" ' for '"' with &ft=vim (comment start)
Exclude delimitMate on "unite" filetype
Remove call to s:option_init("excluded_ft"): it is a global setting only