Make grid margins automatic. Makes the whole grid to fit the available content instead of aligning its inner text.
Refactoring for better optional  support of prefixes.
Additional refactoring to better deal with prefixes
Initial visibility functions
Consistent way to deal with no prefix without especial "all" medium.
Visibility rules
List example added and gutter tweaking
Media queries as variables to make it easy to manage breakpoints
Updatd examples