Update _block-grid.scss

Adds the variable $block-grid-media-queries to control whether block-grid class styles are processed. For those who are only using mixins to define grid behavior (semantic HTML junkies), this may be set to false to lighten the output file by ~3kb.

Also, this is consistent with the _grid mixin behavior in Foundation 4 in which the user defines media queries. For example, if I '@include block-grid(5);' in my SCSS document, I can simply add '@include block-grid(1);' in my '@media #{$small}' breakpoint.