Add support for font options

[ShahriarKh]

Since the theme is using Source Sans as the default font, I think it will be cool to add some options to toggle font features. For example:

* {
    font-feature-settings: "ss04", "ss03"; /* alternate "g" and "a" glyphs */
    font-variant: slashed-zero;
}

This can be done by some simple checkboxes in the Style Settings plugin. Here you can see all available options: https://beautifulwebtype.com/source-sans-pro/

[ShahriarKh]

I noticed that this is added in the latest version. Awesome 🔥.