More work on decoupling the general runtime from Interpreter. The goal
is becoming clearer. Interpreter should be one possible way to execute
code inside a VM. In the future we might have other ways :^)
Currently only supports setting the foregound and the background colours.
This patch also unifies `foreground_color' and `background_color' used
throughout to a `Format' struct, in hopes of getting more formatting
options one day :P
This commit adds a generic interface for cell types and hooks it up.
There is no way to set these from the UI, and so they're not saved
anywhere yet.
Also implicitly converts numeric values (strictly integers) to numeric
javascript values, as numbery-looking + numbery-looking === string is
not very interesting. :^)
