text halo option

[Fil]

tbd:

• The stroke: white is controversial, since we don't know that currentColor is black (or any dark color).
• API design question: do we need three options (halo: true, haloStroke = "white", haloWidth = 4)?

closes #666

[mbostock]

Another possibility is that we could add to #701 such that if a text mark has both a stroke and a fill, the paintOrder defaults to “stroke” (so that the stroke is drawn underneath the fill, rather than vice versa). That’s a little bit opinionated, but it means we don’t need a dedicated halo option, and we require the user to specify what the color will be. We already default strokeLinejoin to round, so you’d probably also want to specify strokeWidth… otherwise you only get effectively a 0.5px halo. But either way it’s a lot easier than having to duplicate the entire mark!

[Fil]

superseded by #703