Implement support for the noop command

The noop command does nothing, but allows for untagged data to be
returned. Implement support for it in the IMAP client class.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>
Use case insensitive command names

There's no reason for the commands in mxsh to be case sensitive. Use
case insensitive comparison, which will make it possible to use a
combination of both upper and lower case letters.

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