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