Inspired by SassyJSON and o-grid. Working in harmony with sass-mq.
Useful to provide the breakpoints list to the JavaScript.
$mq-breakpoints should be a map of breakpoint like in sass-mq.
Use @include mq-expose(); to provide the current breakpoint through body::before and expose the list through html::before to the JavaScript.
@function mq-json generate a json with the breakpoints list.