Support for more notify levels

[trygveaa]

Feature description

In Slack there are message threads, which are messages in reply to a specific message. Normally these are in a separate view from the channel, but slack.py has an option to display these messages in the channel so you can view all the messages without having to open the threads. These messages are displayed with a prefix so you can see that they belong in a thread (and which thread they are in).

When a new message is sent to a channel or thread it's added to the hotlist. Since they are both in the same buffer with the option mentioned above enabled, it would be useful to see in the hotlist if the buffer got a new message directly in the channel or if the message was in a thread. Having more notify levels to use in the hotlist would allow for that.