Add :disabled state to button placeholder

This brings the functionality that previously existed in Bourbon
into Bitters.

Closes #87

Reference:

https://github.com/thoughtbot/bourbon/blob/dd86698/app/assets/stylesheets/addons/_button.scss#L51-L54
Merge pull request #91 from jeffbyrnes/fix-button-disabled-state

Add :disabled state to button placeholder