Support gAMA, cHRM chunks

**Is your feature request related to a problem? Please describe.**

Attempting to read an image with a `gAMA` and `cHRM` chunks ([which are in the PNG1.2 spec](http://www.libpng.org/pub/png/spec/1.2/PNG-Chunks.html#C.gAMA)) results in `UnknownChunkType` error.

**Describe the solution you'd like**

I'd like to be able to losslessly stream a PNG from a `Decoder` to `Encoder`, to do so it would be ideal to propagate all chunks from the source to the destination. Perhaps there could be an extra variant `Unknown` added to `Chunk` that carries the chunk type and data, so that it can be passed through to the encoder as-is.

**Describe alternatives you've considered**

I can ignore the error for now and skip this chunk.

**Additional context**

The image in question: