Add support for latest pre-release

As a maintainer of an application with a stable and beta channel, it would be great if I could somehow tell the API to return the latest prerelease with a parameter (maybe something like `https://update.electronjs.org/{name}/{repo}/{platform}/{semver}/beta`) and to otherwise return the latest stable release.

Would make for a very simple release cycle where releases can be marked as pre-releases, picked up and tested by those on the beta channel, and eventually bumped up to a full release on GitHub.