Implement -centered placeholder to use with grid-column
Implement a primary-button-color and use a separate placeholder for button tags

(To be detailed in Canvas)
Move grid-column-centered placeholder into the generator mixin
Create -centered suffixed placeholders within the main grid generator mixin
Rename default button placeholders
Move primary color to the buttons component
Remove unused variable
Alphabetical ordering :cop:
Merge branch 'feature/ap-tweak-button-grid' into develop
Bump version to 0.4.0