Add flexbox and viewport units support

[morteza]

CSS flexbox and viewport units are all semantically sensible, and seem fit well in this project.

[jlukic]

Yes! Yes! Yes!

Flexbox will come. Ideally each UI definition will be able to have multiple 'implementations' which is part of what im working on now. So people could download a flexbox grid for example instead of an EM based grid.

[brigand]

@jlukic if you want decent browser support, you'll need to have everything duplicated to support EM where VH and VW aren't supported.

.foo {
    width: 10em;
    width: 1vw;
}