Consolidate all buttons under one component

This closes issue #12. 
I only need to apply the `border-color` property as that's the only part of the border that changes. `border-width` and `border-style` don't change therefore they're inherited from [`btn--main`](https://github.com/chris-pearce/scally/blob/consolidate-all-btns-into-one-component/components/_button.scss#L128).