Send IMAP list command

Parts of parsing the list response has already been implemented, but we
can't send the command. Implement support for sending the list command.

IMAP requires mailbox names to be encoded with a modified version of
UTF-7. We don't do this right now, and it should be added at one point.
Until then not all mailbox names might work with the list command.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>