Add ability to define custom breakpoints

(Ab)use of Sass lists ahoy.

Using the $breakpoints variable users can define media groups in case they
need additional ones or would prefer a different naming convention to
the default provided.

This is slightly easier than declaring new breakpoints at the end of the file, and helps enforce the line between "customize the variables above this line" and "you won't need to modify anything below". Initial breakpoint declaration is slightly more verbose, but is more transparent and gives the user more power to customize them.