Added placeholder selectors for .copy-size classes

Using `.copy-size`, `.copy-style` and `.copy-baseline` is a bit of a code smell. Much more so when you introduce entire wrapper elements just to hold those classes.

A much better approach is to `@extend %copy-size` (and so on) to make the relevant parts of your page display as styled copy content. In Blloon I'm phasing out usages of the `.copy-*` classes, and their wrapper divs, and wherever possible any other wrapper divs (`.container`, `.ll` and so on) to avoid situations like this:

![stupid nesting](https://cloud.githubusercontent.com/assets/12165/3919832/6b46866a-23ae-11e4-84e2-4f678ae5eb7f.png)