Allow extra keyword arguments in the Transcoding protocol

[nhz2]

This PR adds the ability to extend the Transcoding protocol in a non-breaking way.

This is desirable to support non-streaming codecs better. Ref: JuliaIO/CodecZstd.jl#46

To do this I added two new optional functions to the protocol: process2 and startproc2.
These are equivalent to their previous versions, except they can accept any keyword arguments.

These are optional to maintain compatibility with existing codecs.

This PR also sets the mode to :transcode instead of :write when calling startproc in unsafe_transcode!
This is to allow non-streaming codecs to error if they are used in a streaming context. Ref: JuliaIO/CodecZstd.jl#46

Edit: I'm removing the :transcode mode change because this can be a different PR.

[mkitti]

Can you give me an example of how this would work?

[nhz2]

I'm working on a CodecCBlosc1 to see how new keyword arguments could be used.

The goal is to allow a compressor Codec to avoid buffering input if there is no more input in a frame and enough output space is available.

One thing I found is that any input passed to process needs to first get passed to minoutsize, so I'll need a minoutsize2 as well.