Add font-display property

[ObserverOfTime]

This pull request adds a variable which is used in the font-display property and defaults to swap.

Here's a usage example for each syntax:

/* CSS */
:root {
  --font-display: block;
}
@import url("fira.css");

/* SASS */
$font-display: block
@import "fira.sass"

/* SCSS */
$font-display: block;
@import "fira.scss";

/* Stylus */
$font-display = block
@import "fira.styl"

/* Less */
@import "fira.less";
@font-display: block;

I also took the liberty of fixing the SASS & Stylus imports.