Add a pipe-pane command to allow a pane to be piped to a shell command, for

example:

	pipe-pane 'cat >~/out'

No arguments stops outputing and closes the pipe; the -o flag toggles a pipe
and on and off (useful for key bindings).

Suggested by espie@.

cmd.c

@@ -71,6 +71,7 @@ const struct cmd_entry *cmd_table[] = {
 	&cmd_next_layout_entry,
 	&cmd_next_window_entry,
 	&cmd_paste_buffer_entry,
+	&cmd_pipe_pane_entry,
 	&cmd_previous_layout_entry,
 	&cmd_previous_window_entry,
 	&cmd_refresh_client_entry,