flexbox version of css

[dkebler]

@liwenyip I'm getting closer to launching my landing page starter. https://dkebler.github.io/landingpage-guide/ I'll incorporate your shortcode (see plugins in the guide). My landing page generator is 100% flexbox and BEM so I'll write you a flexbox version of your css. Might talk you into modifying the shortcode a bit to add some other BEM sub classes but your main wrapper uses the same class "box" that I use to wrap anything in a flexbox so that's good.

[liwenyip]

I'm not really familiar with flexbox or BEM, but happy to accept a pull request.