wish: borders-only variant

[almccon]

There are many times (especially with Map Stack) where you might want the borders from toner, but without roads. Especially if you're using Toner to put labels on a Watercolor map, which already has roads but lacks borders. Adding toner-lines or terrain-lines on top of Watercolor results in unpleasant thin road lines down the middle of the nice thick watercolor roads.

Example (in this case using terrain lines and toner borders):

Similarly, there are certainly times where people have asked for versions of toner-lines without borders.

This is also all true for the Terrain map, so this issue is very similar to stamen/terrain-classic#121