.archivemailrc support

When you get a file-not-found in the 6th mailbox of 10, it aborts the whole
run. Better to fail gracefully and keep going.

Think about the best way to specify the names of archives created with
possibly an --archive-name option.

Add more tests (see top of test_archivemail.py)

We need some better checking to see if we are really looking at a valid
mbox-format mailbox.

Lock any original .gz files 
- is this necessary?

Check for symlink attacks for tempfiles (although we don't use /var/tmp)

Add an option to not cut threads.

Add MMDF mailbox support

Add Babyl mailbox support

Add option to archive depending on mailbox size threshold 
- is this a good idea?

Add option to archive depending on number of messages
- is this a good idea?