Add ability to specify pty width/height/term

[eddawley]

channel.get_pty() accepts width/height/term but spur does not provide any way to give them. This can cause some interactive apps (eg curses ones) to use the default dimensions which are quite small. TERM can at least be used in place of setting it on the pty but there is no easy equivalent for dimensions. Please add some mechanism to set the dimensions so that a caller can set them to match the current terminal's.