urlselect: support weechat command in `bind`

if the second parameter for `/urlselect bind` starts with #, it will
be treated as Weechat command. for example, to bind /pastebuf command
on the selected URL to key <3>:

    /urlselect bind 3 #/pastebuf
pastebuf: readme++ and remove screenshot

Y U COMMITTED SCREENSHOT? BAD SCREENSHOT! BAD!
also removed the buffer_switch_cb. it was never used.