Bugfixes:

- Fix panic while decoding some images, has no precise cause in the file.

- Warn about `set_extensions` being unimplemented...

Features:

- Added `StreamingDecoder::version` to query the precise version of the

standard used for encoding the file. This is merely a hint.

- Added `DecodeOptions::allow_unknown_blocks` to skip over unknown or

unspecified block kinds.

Optimization:

- `Frame::from_rgba` now recognizes when less than 256 colors are being used,

dynamically skipping the quantization phase.

- Encoding image chunks is faster and simpler