Highlighting support

[peterbrittain]

I've just found out about https://github.com/chjj/blessed. This is a node.js library that aims to provide a similar high-level API to terminals as asciimatics. It has some neat extra features that it would be good to add.

The most obvious is the background colour highlighting that it puts on the main page demo. This provides a mechanism for painting the background with a new solid colour or a 50% linear combination (of the RGB colour tuples) of the existing BG colour and the highlight colour.

[peterbrittain]

Added basic highlight() method to Screen. Still need to add blending support, though. Looks like I should go for a weighted approach - see http://stackoverflow.com/questions/1847092/given-an-rgb-value-what-would-be-the-best-way-to-find-the-closest-match-in-the-d

[peterbrittain]

The nature of this highlighting is that to be fully effective, I need to resolve the full Screen refresh tracked in #24. In the meantime, it is possible to force redrawing of the Screen by explicitly clearing it for any effects/widgets that use this feature, but this is sub-optimal to say the least.