Add configurability of "quote character" for JSON factory

[cowtowncoder]

Although both byte- and char-backed generators have internal setting for quote character to use (defaulting to double-quotes), there is currently no way to change it to anything else (such as single-quote (apostrophe)). But with 2.10 it should be possible and easy to allow configuring it.

[cowtowncoder]

Initial version working but need to work on escaping aspects. Chances are that supporting proper escaping for String values turns out to be easy, but not so for Keys (since code is heavily optimized, uses pre-construction).

Also: currently limited to ASCII range for UTF-8 backed output: need to add some sort of validation, and probably just not allow for either backend since escaping of arbitrary character gets complicated even with char-backed (considering existing implementation).
But this should be ok as the main use case really is to support apostrophes as alternative, not so much "any character".