Add support for :rmdir

The `:rmdir` command removes the current directory (`-f` is required if
the directory is not empty).

This is not supported on the notmuch backend.

An issue with the maildir backend is that some sync programs (e.g.
offlineimap) may recover the directory after it is deleted.  They need
to specifically be configured to accept deletions, or special commands
need to be executed (e.g. `offlineimap --delete-folder`) to properly
delete folders.

A danger of using this on the IMAP backend is that it is possible for a
new message to be added to the directory and for aerc to not show it
immediately (due to a slow connection) - using `:rmdir` at this moment
(with `-f` if the directory already contains messages) would delete the
directory and the new message that just arrived (and all other
contents).  This is documented in aerc(1) so that users are aware of
possible risks.

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