Allow more granular settings

i.e. on a per-font basis. Can now specify version, hinting, kerning and charset, e.g.

```
@include guss-webfonts(
    (
        'Guardian Sans Web': (
            (weight: 'regular',  style: 'normal', version: '0.2.0', hinting: 'off', kerning: 'off', 'charset: 'ascii'),
        )
    )
);
```