Should explicitly set `border-box: content-box` for :before/:after, if needed

[jkjustjoshing]

I'm trying to use icono-chain with a Bootstrap-style CSS reset. One part of this is:

* {
    box-sizing: border-box;
}

However, it seems that icono assumes the box sizing on the :before and :after elements will be content-box (browser default). Since setting the box-sizing of every element to border-box is fairly common, content-box should explicitly be set on [class*="icono-"]:before, [class*="icono-"]:after.