[css3-flexbox] multiline

IE10 preview has implementation of multiline flexbox (in 2009 syntax).

Since there is an implementation, multiline should get be represented in current drafts. It can be lower priority, optional or CSS4, but it should be there unless we object to functionality in principle which I don't think was ever the case.

Does it sound reasonable? I can suggest new properties and values to better match new syntax.