Put supported formats behind feature flags

[freijon]

I'm a Gentoo user and we like to enable/disable features at build time, reducing the dependency tree and compile time. That's why I think it could be neat to make each supported format a feature that can be enabled/disabled. Alternatively, if you think that's overkill, you could also define some "core-formats" that you decide are a fixed part of ouch and make niche formats a feature. That way users don't have to install the libraries of all supported formats.