More robust flag options in frontend

[bbworld1]

Right now, each flag on the nunaweb frontend acts independently of each other:

It might be good to have some flags be capable of disabling others (e.g. disabling serialization support should probably also disable the float serialization flag, as AFAIK it has no effect if serialization support code is not generated).

[pavel-kirienko]

This is sensible. Also, the option flags should be language-specific, because, for instance, the Python backend is not configurable at all: it is always any-endian (big/little is chosen at runtime) and full serialization is always enabled there.