Imap idle

Ok, another and last one today I hope.

I really missed IDLE extension as this is main reason why I had been looking for this program.

Here is my attempt to do it correctly but I want to test it further. As previous commit, I'm sending pull request mainly to notify you about this attempt and to collect your ideas and comments.

I'm not sure yet how it will handle computer suspend and imap timeout but I'll test it and improve it as I'm using it to fetch my work emails :)

IDLE is only used when enabled with -i option.

I think it will be great to do check_all_folders every X seconds even when using IDLE but need to think about locking or something like it.