Fix for #955: Stop weird behavior when adding percentages

What do you expect the result of `10% + 10%` to be? Well, everyone in
the world would say `20%`. Stylus thinks it should be `11%`.

Fixes learnboost/stylus#955
Why is it illogical? The behavior would be different if we remove the block entirely.

The `if`-case does the percentage calculation when the second operand of an addition or subtraction has the `%`-Type.

By adding the first part (`'%' != this.type`) we skip the calculation only when _both_ operands are `%`-Type. I might be overlooking something tough.