`idlefolders` needs but does not set `autorefresh`

[madduck]

The example config for idlefolders says:

In between synchronisations, Offlineimap can monitor mailboxes for new
messages using the IDLE command. If you want to enable this, specify here the
folders you wish to monitor. IMAP protocol requires a separate connection for
each folder monitored in this way, so setting this option will force settings
for:

• maxconnections: to be at least the number of folders you give
• holdconnectionopen: to be true
• keepalive: to be 29 minutes unless you specify otherwise
• singlethreadperfolder: to be true

The setting autorefresh is not mentioned, but under holdconnectionopen, it
says:

This setting has no effect if autorefresh is not set.

… and indeed, idlefolders do not work unless autorefresh is explicitly
set. I think this setting should be similarly forced like
holdconnectionopen.

[madduck]

Nice. I commented the pull request, which is a nice start, but not enough.