New Printer-Friendly Style

Based on a handful of requests, I've added a new style, "Print" (a.k.a. "print" in the API) which is better suited for color printing than either "Poster" (which consumes black ink) or "Atlas" (which is grayscale-only).

The colors for routes, borders and bases should be left alone unless they're close to white, in which case a default color will be used, unlike "Atlas" style where fixed colors are used everywhere. This was a bit of a rush job so I may have missed a few cases (where I need to explicitly override a color) or introduced bugs, so let me know if something doesn't look quite right.