This PR is addressing #220. It's not super important but it's a breaking
change, so if it's something we want to do then I thought I should get
it in now before the next release.
- [x] pretend can become another variant of OutFile, probably
OutFile::None, as that's what it essentially is - just another output
destination and not a separate option
- [x] ~~backup and~~ preserve_attrs should become properties of
OutFile::Path variant (so that it would contain Path { path, ~~backup,~~
preserve_attrs }) as they don't have any effect on any other output and
so semantically belong there best
Closes#220
* Refactor ColorType
Move transparency and palette data into the ColorType
* Fixup tests
* Make more use of helper functions
* Change BitDepth to u8 representation with TryFrom
* Fix clippy lints
* Don't use unstable language features
* Restore documentation on transparency/palette
* Refactor filters as enum
* Include filter byte in filter output
* Add entropy filter
* Add bigrams filter
* Add bigram entropy filter
* Add brute filter
* Replace bit-vec
* Add tests and benches
* Show filters in help
* Use FxHasher in color to palette
* Use windows function for minor improvement
