Add grunt-webfont task.

On previous projects, we have used a custom grunt task for running fontcustom to generate icon fonts.

This adds the [sapegin/grunt-webfont](https://github.com/sapegin/grunt-webfont) task to generate icon fonts.

This will also require some external dependancies to be installed in order to run the task.

```
brew install fontforge ttfautohint
```

Ideally, this task would be run only when the icons change and the compiled scss and fonts would be checked in to the project repos. This way, these dependencies will not need to be met by backend developers to build and deploy a site.