interrupt an IDLE connection manually

interrupting the IDLE command might also be useful when the same IMAP connection is used for doing some jobs (move, markseen, whatever) as well as for IDLE. in this case, when a new job is queued, it is great if the IDLE command is interruptable.

this issue seems to be similar to https://github.com/jonhoo/rust-imap/issues/121 (cancel idle connection)

just had a look how this is done in libetpan, seems as if they're doing it by writing a single byte to the stream https://github.com/dinhviethoa/libetpan/blob/master/src/data-types/mailstream_cancel.c#L173 (for mac, they use an additional approach, https://github.com/dinhviethoa/libetpan/blob/master/src/data-types/mailstream_cfstream.c#L1183 )

not sure if this is doable in the same way, however.

cc @hpk42