added ctrl-X option to close current channel window

Hey @robbinsr,

This pull request adds an input **ctrl-X** which closes the current channel window.

**remove_channel(void)** edits the doubly-linked list to remove the current channel, then sets the current channel to the its *prev* channel. If curr is the only channel, this will have no effect. Conversely, if curr is not the current channel, the *removed* channel will be freed.

So, **ctrl-X** calls remove_channel(), then draw_full() to re-render the client.