Make property renaming optional

I am integrating Flatbuffers with [existing data standards](https://spacedatastandards.org/), it is mandatory to use the exact property names / namespaces contained in these standards going forward, including all capitalization.

I've hacked in a quick fix to remove [`MakeCamel`](https://github.com/google/flatbuffers/blob/master/src/idl_parser.cpp#L98), [`MakeScreamingCamel`](https://github.com/google/flatbuffers/blob/master/src/idl_parser.cpp#L112), and [`MakeCamelCase`](https://github.com/google/flatbuffers/blob/master/src/bfbs_gen.h#L96), as well as disabling this [warning](https://github.com/google/flatbuffers/blob/master/src/idl_parser.cpp#L753). I can work on a pull that puts disabling these behind a flag, for users in my situation, if it is something that would be accepted.