Make prompt dynamically generated.

Prompt is now generated by a promptf() call for every cmdloop iteration.
We make the prompt aware of extruder temperature.

In order to do this, we consolidate handeling of printer status
with a Status class. The status class is updated by recvcb.

This has the side effect of simplifying the implementation of gettemp.

We also detect whether there is a heated build platform or not, and
don't display info about it if there isn't.
Make the dynamic prompt temperature stuff configurable.
Merge pull request #313 from colah/temp-prompt

Temp prompt