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 **"("**.