Add option to remove before-close characters

4c4902f7196ee3ef0049a705fe93cc91cc0d4d6a improved the on deciding when to close
a pair. The choice that closing pairs is OK before any `b:AutoClosePairs` value
means that pairs will close ahead of strings. For example, putting `(` in front
of `"` gets you `()"` instead of `("`.

`AutoCloseDoNotCloseBefore` allows setting an explicit set of characters to
never close in front of.