Restore font-family to :before selector.

When Font-Awesome is used with other third-party css framework,
like https://github.com/hakimel/reveal.js, said css framework
can define (wrongly or rightly) global rule like
https://github.com/hakimel/reveal.js/blob/master/css/reveal.css#L16
that end up overriding font-awesome.css rules.

And that, even if font-awesome.css is declared _after_ all other css.

Restoring that font-family in the :before selector ensure that rule
is present in the final css rules applied to an html element.

Similar to #1716