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.
Not sure I agree with this name. I usually understand the file extension to the the letters following the filename.

i.e. scss is the file extension of _variable.scss