Add general-purpose formatFeatures methods for parser, generator

[cowtowncoder]

Parsers and/or generators that handle non-JSON content often have additional features beyond core JSON features (and perhaps JSON format itself may, in future?).
Adding these in sub-classes for parser, generator, and JsonFactory works acceptably, but becomes problem at databinding level when trying to support per-call overrides for particular formats.

To support "general but specific" config features it is probably necessary to add a small set of supporting methods, so that ObjectWriter and ObjectReader can support overridability. This may include things like:

• Introspection method for checking feature enum for format impl (to guard against mismatches)
• Parser/Generator can have generic override (flags to set/clear with separate mask), possibly accessor
• Parser/Generator base class can have format-feature field
• ObjectReader/ObjectWriter can have methods for specifying overrides.

[cowtowncoder]

I wish I had more time to get this ready for 2.4 (to be used by 2.5 modules); but as things are this won't be happening in time.

[cowtowncoder]

Superseded by #196 (related but not exactly the same)