Add "reset" escape sequence

This resets the fore- and background colour (there's not much more to
reset, so... well, yeah).
Add value-to-colour function

It represents the passed value as a colour. It's basically a function
that makes a gradient red, orange, yellow, green, from 0 to 100.

In order to get the inverse (e.g. for displaying the memory usage, where
0 is really, really good, and 100 is really, really bad), simply pass
100-[value] to the function.

The function has been created through trial and error. There is no
higher magic behind this (and 51 doesn't mean anything).

Please move on.