Improve the typography configuration

[titouanmathis]

Proposal to improve the typography configuration:

• All classes are generated by Sass maps which can be configured at will
• The font families are generated by a map, and their webfonts are specified in a child map with the following structure:

$type-fonts: (
  serif: (
    name: Georgia, // Used in the @font-face declaration
    stack: 'Georgia, sans-serif', // Used in the font-family declaration
    webfonts: (
      (
        filename: 'georgia-regular',
        weight: 400,
        style: normal,
      ),
      // ...
    ),
  ),
  sans-serif: (
    name: Arial,
    stack: 'Arial, sans-serif',
    webfonts: (
      (
        filename: 'arial-regular',
        weight: 400,
        style: normal,
      ),
      // ...
    ),
  ),  
) !default;

[titouanmathis]

Will fix #13 🎉