Refactors, new features, etc.

Added new border types, refactored that code a bit, a few extra methods and options, and moved some of the text converting/stripping into functions, so I can use them in the CLI app I'm working on.

I'm confused by `Scrollbox`, btw.  It uses `this._clines` in many places, but _clines is only set in parseContent, which is only run on screens (because `detached` is only true for screens (?)).  I'll look into it more, but maybe you can smoke screen that for sanity.

Thanks for blessed, it's friggin' awesome.