Map ; and , as "all-or-none".

Before this change it was possible to have , and \ _both_ mapped to
<Plug>SneakPrevious in _different_ modes (eg, "\" in normal mode and "," in
visual and operator mode). That's really unlikely to be desirable. Instead,
either map _all_ modes _or_ require the user to specify a explicit mappings.
fix: highlight color is muddled on 256-color terminals