support serialization target/format extension via Legolas schema version extension

[jrevels]

Right now Onda.jl has its own system for supporting author-defined extensions for new serialization formats (xref #97). I think it'd be nice/tractable to just use Legolas for this instead.

e.g. define RawLPCMSignalV1 > SignalV2, LPCMZstSignalV1 > SignalV2, etc.

this would be a breaking change though, and i didn't want to stuff #133 with more breakages

[jrevels]

If Legolas supported multiple inheritance this same approach could be used for (de)serialization targets, i.e. the sample data storage system, in composition w/ the format extension idea above.

so you could have MySignalV1 > (LPCMZstSignalV1, S3SignalV1) > SignalV2, or MySignalV1 > (RawLPCMSignalV1, LocalFileSystemSignalV1) > SignalV2, etc.