Teach the bot to recognize !ao < FILE.  #14.

This commit also teaches the bot to differentiate between !ao URL and
!ao < URL.  (Cinch triggers *all* matching command handlers, not just
the first or last one registered.)
Teach pipeline how to handle a file containing URLs.  #14.
Stream URL file from source; write file as we receive it.  #14.

This shifts responsibility to wpull for parsing the URL file; all we do
is fetch it.

Potential issue: I'm not sure if Response.iter_content bypasses
transfer-encoding handling.  If it does, then we'll have to use a code
path that doesn't.  (Another possibility: just shell out to curl...)
Teach the bot how to queue !ao < jobs.  #14.