ESRG as a Sass Mixin

@johnpolacek - I've updated the ESRG Sass to decouple the defaults from the top of the file, while morphing ESRG into a mixin called gridGenerator();

See example-grid.scss for usage.

```sass
@include gridGenerator()
```

I've also added support for EMs

```sass
@include gridGenerator((0,35em,55em), 'em')
```

Example to override all the defaults

```sass
@include gridGenerator(
  (0, 35em, 55em),   // breakpoints
  'em',              // breakpoint type 'px' or 'em'
  18,                // number of grid columns
  1em,               // outer padding
  1.5em,             // inner padding
  (sm-,med-,lg-)     // breakpoint prefixes
)
```

Happy to make changes, but thought I would let you update the demo/documentation page.