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.