bot: Experimental !a < FILE mode.

This is intended for archiving Meetup Everywhere sites.  Most of the
sites in Meetup Everywhere are very small; therefore, if we were to
spawn 6,000-ish jobs, there will be a lot of wasted effort in

1. managing multiple jobs
2. duplicate asset files

!a < FILE gets all sites in one set of WARCs and could end up being more
time- and space-efficient.