add commands to rebuild failures of certain nightly versions

[syphar]

from #301

Add a command to the cratesfyi binary that queues up builds for all crates which were built with a specific version of rustdoc, at a lower priority than previously-failed builds. Even though this is a manual action, it allows us to proactively rebuild crates that have issues like #280 without manually rebuilding every crate that used that version (and again, without choking new releases out of the build queue).

This is a thing that I have to regularly do directly in the database after nightly problems.

We typically free the nightly version in these cases so the builds continue running.

What I would like to have is a CLI subcommand with which I can queue rebuilds for all failed builds of a certain single nightly version, or a date-range of nightly versions.

[GuillaumeGomez]

I strongly approve the idea!