Syntax-specific settings replace defaults, gh-17

Rather than merging syntax-specific settings with the global ones,
replace them.  This means if you just want to add an operator to a
language, you'll first need to copy Abacus.sublime-settings, then
modify it accordingly.  If you want to remove or customize a token's
behavior in a specific syntax, then you can now do so.

I've combined Abacus.sublime-settings with the existing Ruby one (&
the new PHP one) since that is the intent for those languages.

I've left the CSS one with just the ":" token.  It's possible some of
the default tokens should be copied over as well, but ":" is
undoubtedly the main one.