Add (min|max) resolution to breakpoint-string-value()

Right now when I use `@include breakpoint(resolution 1.3dppx) {}`, the feature and value are switched by lines 256+ in _breakpoint.scss:

          @if breakpoint-string-value(nth($breakpoint, 1)) == true {
            $feature: nth($breakpoint, 1);
            $value: nth($breakpoint, 2);
          }
          @else {
            $feature: nth($breakpoint, 2);
            $value: nth($breakpoint, 1);
          }

Adding resolution, min-resolution, and max-resolution to `breakpoint-string-value()` should fix it right up. If I'm missing some obvious way to use resolution media queries and causing my own problems, let me know.

**[Edit]** I see that in the hi-res example using `device-pixel-ratio` you reverse the feature and value (`2 device-pixel-ratio`). What's the reasoning behind having some expressions in normal code order and some reversed?