Temp prompt

By making the pronsole prompt be generated every step of `cmdloop` by the function `promptf`, then prompt can display information about the state of the printer. In this case, we are displaying information about the extruder temperature, but it could easily show percent of the print done, etc. This saves people needing to keep running `gettemp`.

In order to facilitate this, information relating to the status of the printer was abstracted into a Status class. This made simplification of the `do_gettemp` code a very natural consequence.