Allow for Sass variable overriding when including stylesheet

[martinbean]

I use Bootstrap in a lot of projects. In their _variables.scss file, they suffix each variable value with !default. This means users can include the main Bootstrap style sheet, but define any variables they wish to override beforehand:

$brand-primary: #c00;
$font-family-base: "Ubuntu", sans-serif;

@import "node_modules/bootstrap/scss/bootstrap";

It would be good if the same approach could be taken by Sir Trevor so I can define my custom variable values and then include Sir Trevor’s styles in my application stylesheet:

@import "variables"; // My custom Bootstrap and Sir Trevor variable values

@import "node_modules/bootstrap/scss/bootstrap";
@import "node_modules/sir-trevor/src/sass/main";