feat(mpegts): expose raw audio/video writers

[kevmo314]

I am working with elementary streams that are already processed for transport (eg h264 already in annex b format) and would like to call the muxer directly.

[aler9]

Hello, the MPEG-TS muxer inside this library is currently intended for generating streams from unwrapped frames.
The exposed methods are intended for this and are immutable, their signature won't change unless a major version is released.

Exposing inner methods would allow a series of use cases that are unsupported and untested, and would make impossible to eliminate these methods in the future (due to refactoring) for the reason explained above.

If you want to access inner methods, you can either fork this library or work directly with go-astits.