Rename message::from() to message::first_from()

This emphasizes the fact that this only returns the _first_ "From"
field in the message.  The general idea here is that there may be more
more than one of these but all we're really getting is the first one
for speed reasons.
Make the SQL "To:" header availiable

This is what's supposed to be used during "mhng-post".
Prefix the other SQL-stored headers with "first_"

This emphasizes the fact that we're only obtaining the first header of
this type, not all of them.
Add the "numbers" parse flag

The general idea here is that some commands (mh-mime and mh-urls)
actually just take arbitrary numbers as their input instead of
sequence numbers.  This new flag allows this to work fairly easily,
and to work for all the cases where I need it.
Implement "mhng-mime", a MIME parser

Note that this isn't fully implemented yet because I don't have MIME
Content-Transfer-Encoding working, but that's going to take a while so
I don't want to deal with it for now.
Implement "mhng-show"

This also has problems with decoding messages, just like the previous
commit does.  This is largely because I just haven't bothered to deal
with any of this yet, as I don't have any time...