Selector prefix mixin

I have added one file (containing two new mixins) named "_selector-prefix.scss" in the "app/assets/stylesheets/addons" directory.

A wee bit of background: I use the Modernizr (modernizr.com) library, which appends classes to the html element depending on your browser capabilities.  Occasionally, I need to modify styles according to those appended classes.  I have developed these two mixins to abstract that process.

Example SCSS file:
```scss
.example {
    color: green;

    .child {
        color: blue;

        @include class-prefix(no-js, display, none);
    }
}
```

becomes:
```css
.example {
    color: green;
}
.example .child {
    color: blue;
}
.no-js .example .child {
    display: none;
}
```

I hope this helps.

- Kevin Weber