Dependency on pylibjpeg-rle

[hackermd]

Why does this package depend on pylibjpeg-rle?

1. Run-length encoding is not a JPEG codec.
2. The pylibjpeg-rle requires a Rust compiler, which is not widely available.

I would suggest making pylibjpeg-rle (and probably also pylibjpeg-openjpeg) an extra install requirement (using extras_require).