Add ability to keep a rustfmt-based variable number of nightly copies

[k3d3]

In one of the new rustup versions, nightly releases appear to be skipped if a built rustfmt doesn't exist within it. Because of this, if a mirror only keeps one/two/more nightly copies and none of them contain rustfmt, rustup will loop (and fail) through every nightly version back to 2014.

Add a parameter in the config that only purges old nightly copies once a version containing rustfmt is downloaded.