make maps readable for color-blind users / in greyscale version

[christianvoigt]

as pointed out in the Hacker News discussion:

The map view relies on color for information, which makes it hard to read as a colorblind user. -- dharmab

The default red/green background colors should be lightened dramatically, eg ffcfcf and cfcfff. The colored arrows should just be black imo. People will fuss over red green color blindness as well. May want to add a small visual indicator that can be seen in a grayscale version, like a plus or minus above each box, or the words pro and con. -- sagebird

[kopeckyf]

At least some of the ColorBrewer presets that you offer in Argdown are safe for red-green colour-blind users. At least the following are (up to 5 categories):

• colorbrewer-dark2
• colorbrewer-paired
• colorbrewer-set2

However, I'd like to add to this issue request to have grayscale colorSchemes, because they are safe for photocopier use. Many departments only have b/w printers. If people print papers that contain Argdown maps on their office printer, they might not see the difference between attacks and supports. In grayscale mode, would it be possible to change the edges such that supports are marked by solid arrows, and attacks by dashed arrows?