reproduce.tt: Add --help flag

And a --dir flag to override the directory where inputs etc. are
stored.
reproduce.tt: Check for the presence of commands like nix-build
reproduce.tt: Use color
Allow a per-jobset check interval

This allows checking a jobset (say) at most once a day.  It's also
possible to disable polling by setting the interval to 0.  This is
useful for jobsets that use push notification or are manually
evaluated.