This adds `.cron.yml` and a new mach command to interpret it. While
functionality is limited to nightlies right now, there is room to expand to
more diverse periodic tasks. Let your imagination run wild!
MozReview-Commit-ID: KxQkaUbsjQs
--HG--
extra : rebase_source : ddf0a1eadae5a1169c0ead7bcb7b9ce61b255fbf