Add Line Shapes example to docs

- fig : https://plot.ly/~etpinard/436/linear-spline-vhv-hvh-vh-hv/
- inspired by : https://plot.ly/~alex/315/new-line-shape-options/
Maybe a name like "Interpolation Options"? *Shapes* is a little misleading to me. Would all of these be considered interpolations?