Add `font-display: swap;` to generated css file

[mangelozzi]

When performing a Chrome lighthouse audit, the its fails and recommends adding
font-display: swap;

https://web.dev/font-display/?utm_source=lighthouse&utm_medium=devtools#how-to-avoid-showing-invisible-text

E.g.:

@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local(''),
       url('/static/fonts/roboto-v29-latin-300.woff2') format('woff2'), /* Chrome 26+, Opera 23+, Firefox 39+ */
       url('/static/fonts/roboto-v29-latin-300.woff') format('woff'); /* Chrome 6+, Firefox 3.6+, IE 9+, Safari 5.1+ */
}

becomes

@font-face {
  font-family: 'Roboto';
  font-style: normal;
  font-weight: 300;
  src: local(''),
       url('/static/fonts/roboto-v29-latin-300.woff2') format('woff2'), /* Chrome 26+, Opera 23+, Firefox 39+ */
       url('/static/fonts/roboto-v29-latin-300.woff') format('woff'); /* Chrome 6+, Firefox 3.6+, IE 9+, Safari 5.1+ */
  font-display: swap;
}

[Melroy van den Berg]

Must have indeed.

[Mario Ranftl]

Fixed within v1.1.0.