Add ability to use custom stroke colors

[peacog]

Currently if we want to use stroke colors other than the 8 predefined named colors it's necessary to write a style function. It would be great if we could simply pass the hex or rgb value in the color option of the addLayer method. I've created a PR that does that. The color option can be one of the predefined color names, a hex value with or without alpha channel, or an rgb() or rgba() value.

Here's the PR #200

This is an enhancement rather than a bug, but I don't have permission to change the label :)

[symbioquine]

Thanks @peacog! Label updated and replied on PR

[symbioquine]

This change has been merged.