Allow crates to specify which features to build for documenting purposes

[nox]

docs.rs could use a metadata table key to be taught about which features to use when generating the docs. For example, simd has a feature that enables everything in the crate, so that the documentation isn't platform-dependant.

[nox]

Oh, that seems to be #73...

[mathstuf]

@nox I think this can be closed.

[onur]

Yes docs.rs supports building non-default features and metadata.