Add telephone-line segment

[FrozenPigs]

Would be nice to have channel buffers display in a telephones-line segment like they do for in the mode-line.

[wasamasa]

Judging from the contents of https://github.com/dbordak/telephone-line/blob/master/telephone-line-segments.el this looks like something either telephone-line does or the user does. I don't see why Circe would have to support all the different kinds of mode line packages.