Adding support for settings slices as maps, and referring to their keys.

I found breakpoint slicer to be [https://github.com/Team-Sass/breakpoint/issues/103](exactly what I was looking for), except I wanted to be able to refer to my named breakpoints (e.g. sm, md, lg) instead of remembering the numbers of the slices.

I added support for setting a `$slicer-breakpoints-map` instead of just a `$slicer-breakpoints` list.

Thanks for your work!