[css3-flexbox] flex-flow bikeshed

flex-flow seems really complicated. Unfortunately, I don't have anything new
arguments against having so many possible values.

If we're going to have all these options, we should at least be consistent
with writing-mode. Specifically, horizontal-ltr | horizontal-rtl |
vertical-ttb | vertical-btt, should be horizontal-lr | horizontal-rl |
vertical-tb | vertical-bt.