Join new channels on reconnect

[kcm1700]

This is a feature request.

Currently, only configured channels are joined on reconnect.
One can join new channel by calling send_join(), but the channels are not kept when some error occurs.

The desired scenario looks like the following.

1. The library connects to the server and initializes some statuses.
2. A user calls send_join() to join some channels.
3. Something bad happens. Then reconnect() is called internally.
4. The library automatically joins the channels that were joined in step 2.