2.0.0: kerning on/off, WOFF 2.0, various fixes

- [new] Serve WOFF 2.0 format to very modern browsers
- [new] Setting to load fonts without kerning tables:
```scss
/**
 * Kerning
 *
 * - on: larger file, better rendering
 * - off: smaller files
 *
 *  @type String
 */
$guss-webfonts-kerning: 'on' !default;
```
- [fix] SVG fonts work in older browsers
- [fix] `Guardian Weekend Condensed Web` renamed to `Guardian Weekend Cond Web`
to match the internal name character length limit