Symbols!

[mbostock]

Fixes #23.

• [x] symbol.type
• [x] line.interpolate
• [x] area.interpolate
• [x] simpler instantiation of default interpolators (cardinal instead of cardinal()).
• [x] line.interpolate(cardinal(0.5)) or line.interpolate(cardinal, 0.5)?
• [x] rename {line,area}.interpolate to {line,area}.curve? or spline?
• [x] prefix symbols (e.g., symbolCircle, curveLinear)?