Make nightly version configurable

It happens from time to time that some crate can't build on a nightly version which results in broken documentation. In this case the author of the crate usually has to wait for the nightly version docs.rs uses to be bumped and hope that problems will go away.

Right now, my crate barrel fails to build documentation because the diesel version I require requires a newer nightly.

It would be so much better if, apart from bumping the nightly version from time to time, enabling users to tell docs.rs what nightly version to use. This way users can either opt-into the current nightly or pin documentation on a version they know works.

Edit After having seen https://github.com/onur/docs.rs/pull/225 I'm wondering what other work is needed to make the rust version used by docs.rs configurable.

The issue names no files, tests, or entry points. Start by reviewing how docs.rs selects its nightly Rust version and the configuration work discussed in PR #225. Done means users can select or pin a nightly version while the existing default remains available.