Added font sizing mixins and classes to apply heading font sizes to any element.

For example, to display an `h2` as an `h1`:

```html
<h2 class="alpha"></h2>
```

…or, via SASS

```scss
h2 {
   @include alpha;
}
```

This latter example is also possible by explicitly referring to the font size variables, but the mixin has the advantage that it also inherits the `@media` rule and therefore adapts for smaller screens (for h1-h4 anyway).

Let me know if you want me to update the documentation.