Add a conf setting to toggle source and binary caching

As some users may want the binary and source control files regenerated on each
run, this commit adds a setting to freight.conf to allow the caching to be
enabled or disabled by setting CACHE to on or off. It defaults to off, to
preserve the current behavior.
Add freight-clear-cache to remove the cached control files

This adds a simple freight binary written in the style of freight-cache to
allow users to clear the cached source and binary control files.