Added global setting `suppress-responsive` which aims to tame older web ...

PR in response to issue #44.
I like the global variable - but `supress-responsive` seems a bit ambiguous - is there perhaps not a better name for this? Also, it would be convenient to reference Rupture's internal config to this global variable (much in the same way we reference `base-font-size`:

```
rupture = {
   suppress-responsive: suppress-responsive
   mobile-cutoff: 400px
   desktop-cutoff: 1050px
   hd-cutoff: 1800px
   enable-em-breakpoints: false
   base-font-size: base-font-size
   anti-overlap: false
   density-queries: 'dppx' 'webkit' 'moz' 'dpi'
   retina-density: 1.5
   use-device-width: false
 }
```