Support changing release state to Latest with `hub release edit`

[bitfactory-henno-schooljan]

The problem I'm trying to solve: I want to promote a Pre-release release to Latest release (green). However, there are only switches for draft and pre-release types with Latest as the default, which works for creating a release, but not for changing it to the default release type.

How I imagine hub could expose this functionality: Add another release state option to hub release edit, e.g. -l, to signify a Latest release.