Added extra bounds to "bound_symbols".

Added "(" and ")" bounds. This fixes the color highlight
in preproccesors. Without this, first or last hexadecimal
colors in maps, lists, mixins, etc, won't be highlighted.
Like example:

$share-option-color: (shr-link: #9D4EBB, shr-url: lighten(#9D4EBB,5%),
                      shr-tw: #22BBF5, shr-gplus: #B93116);

Here, "shr-gplus" isn't highlighted because their last bound is an
")".
Same happens with color in "lighten(#9D4EBB,5%)", because their first
bound is an "(".