function getBreakpointFromWidth(breakpoints
/*: Breakpoints*/
, width
/*: number*/
)
/*: Breakpoint*/
{
  var sorted = sortBreakpoints(breakpoints);
  var matching = sorted[0];

  for (var i = 1, len = sorted.length; i < len; i++) {
    var breakpointName = sorted[i];
    if (width > breakpoints[breakpointName]) matching = breakpointName;
  }

  return matching;
}
Given a width, find the highest breakpoint that matches is valid for it (width > breakpoint).

@param  {Object} breakpoints Breakpoints object (e.g. {lg: 1200, md: 960, ...})
@param  {Number} width Screen width.
@return {String}       Highest breakpoint that is less than width.