IJsonFormatterResolver extensions for other forms of naming (dash-case)

[jasond-s]

I really would like to be able to mirror the StandardResolvers to implement other forms of naming conventions. I have a need for a dash-case serialiser and it seems very hard to figure out how one might do this with the existing code generation classes all being internal (specifically the DynamicObjectTypeBuilder).

I would be willing to add another StringMutator for this specific use case as a PR, but I feel there ought to be (perhaps there already is and I just can't see it) way to do this using the existing library.