Added !default flags

Currently if you use the `@import "bitters"` statement to load Bitters from the gem or a Bower Components folder it loads _everything_, including the variables file. 

The problem with this is if you want to set your own variables you either need to import all of the Bitters components separately (excluding the variables file). Or edit the variables file itself which isn't ideal when you come to update Bitters in the future.

So the idea is you can set the custom variable values in your own config SCSS file, and import the base Bitters component from the gem or bower folder, and the default variables file will be there as a fallback.

Ref: http://robots.thoughtbot.com/sass-default