clamp HSL format

[mbostock]

Fixes #83.

Note that you don’t get an equivalent color with formatHsl and formatRgb, though, because they clamp in the HSL and RGB colorspaces, respectively. If you want to clamp in RGB colorspace and format in HSL, you have to first format as RGB, then re-parse, then format as HSL:

d3.color(color.formatRgb()).formatHsl()

It’d probably be nice if we had an explicit color.clamp option for RGB and HSL so that you could avoid reparsing. Then you could say:

color.rgb().clamp().formatHsl()