Use underscore to escape HTML entities from stdout

This fixes #80, namely to make sure that HTML entities are properly escaped.

I chose to use underscore because we don't necessarily need all of underscore-plus. I'm amenable to changing this though, as we may in the future want their modifier key map and other utilities.

@hansrodtang, @ciarand - we can continue discussion here. I've checked out ANSI colored output + html entities and all is good.