Add the ability to specify multiple weights and styles in the same font-family

# Update to Web Font Mixin

Status: **Ready to merge**

Reviewers: @jeffkamo @ry5n @kpeatt 

## Changes
- Added a new variable to web font mixin to build out file names based on font style and font weight
- Added tests for web font mixin
- Added the open source Source Sans Pro to the fixtures folder
- Moved icon font to its own folder

## Notes
- This is a backwards incompatible change if anyone is declaring multiple web font includes for multiple different types of font faces. This is probably unlikely but is worth noting.
okay, that makes sense. Except that `$file-extension` is still guaranteed to be an empty string anytime this particular line runs.

That means `$file-extension: $file-extension + '-#{$font-weight}';` is the same as saying `$file-extension: '' + '-#{$font-weight}';`

Might as well make it `$file-extension: '-#{$font-weight}';`

Of course line 55 it makes sense, since there's a chance `$file-extension` might have a value.