Support other sites / providers

Currently only 1 site is supported. There should be a way to switch providers and support for at least a few.

Ideally a handful would be supported out-of-the-box and others available via plugins.
[Tachiyomi](https://github.com/inorichi/tachiyomi) has various providers available via separate repos and APK downloads. I think the latest release is a "clean" version with no providers available out-of-the-box.
Would be cool if provider plugins could be added via something similar to Code Push, that doesn't require a new install. Obviously warnings need to be given up-front about potentially hazardous code distributed by others (alternatively keep a safe list?).

See also #1 - provider plugins are not currently public - Tachiyomi's are though