Remove dependency on default list-sessions format

[voskresenie]

Currently, has_session depends on the default output format of list-sessions being what it is, ie beginning with #{session_name}:. While it's not particularly likely that the default format will change, and the only practical problem with the way it is happens if the input is something like my_session: 4 windows (created Tue Oct 22 07 [1], this seems like a cleaner way of doing it (that's a joke, I watch your vids). But I understand if you would rather leave it as is.

[1] which will match if my_session exists, has 4 windows, and was created Tue Oct 22 between [07:00:00, 08:00:00), in any year that Oct 22 is a Tuesday. But this won't be possible if #5 is merged, since : will be replaced with _