displays child blocks in a responsive FLEXBOX grid

syntax 1 : {up flexbox=x1-x2}content with several child blocks{/up flexbox}
syntax 2 : {up flexbox=x1-x2}content block-1 {======} content block-2{/up flexbox}
x1-x2 are the relative widths of the columns
example : col=4-8 or col=33-66 for 2 columns (equivalent: span4 and span8)