Nesting of spans not working properly

This fails:

```
[highlight]*If we calculate a [klass]*PVector* [var]*f* that is a force of object A on object B, we must also apply the force [var]*f* (or [function]*PVector.mult(f,-1);*) that B exerts on object A.*
```

Working around this now by just removing internal formatting, but would be nice to be able to put it back in eventually.

cc @shannonfry
