!default flag

[pattan]

First of good job.
If I integrates this grid in a project or other framework, there is a need to define variables in a separate file (_vars.scss). It would be great if each variable had a !default flag so I can overwrite with my own values.

@import 'variables'; // own defined variables
@import 'grid'; // default variables

http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#variable_defaults_