Vanilla Radar: Polygon

[photodow]

As of #42 (pull request) the charts-vanilla-radar package only allows for a circle shape. This issue is here, so we can track the progress of the polygon shape within the charts-vanilla-radar package.

See #189 (line number)

What is the expected behavior?
According to the Design Language this is kind of what we're aiming for.

[mhsenkow]

May be fun to include the idea of different line completion types here. see: http://www.d3noob.org/2013/01/smoothing-out-lines-in-d3js.html