Looking through github's style guide, they have the following example code for their icons:
```html
<a href="#" class="button minibutton" icon_class=""><span class="mini-icon"></span>Iconed</a>
```
It's hard from looking at just that example to know exactly where the modifier classes go. Here's the same example code with your suggested ```{class}``` placeholder.
```html
<a href="#" class="button minibutton" icon_class="{class}"><span class="mini-icon {class}"></span>Iconed</a>
```
With the placeholders, it's much easier to see that you need to put the modifier class in two places instead of just in the icon_class attribute or with the other class in the inner span.

I lean towards the ```{class}``` placeholder in the example html.