Implement a --moveto option to move the message to a different IMAP folder instead of deleting it

As per issue #52 (closed), it is helpful for use during testing to be able to move emails to a different folder instead of deleting them. It is also the best way to delete emails on Google Mail / GMail (where EXPUNGE needs to be done for each label, doesn't always work, puts mails in "All Mail" instead of "Trash", etc).

These patches implement this command line and config file option, allow setting the folder to move to, and document the option in the man page.