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
This allows to configure or compile away logging in the library from a single place in Rust apps.
For the CLI side, the usage and output remained the same, except it's now colour-coded.
Fixes#217.
In keeping with being able to use oxipng as a library, this adds a way
for developers to optimize a PNG already in memory, instead of requiring
it to exist on disk first.
