vg,vg/draw: primitives for an Ellipse

unless I am mistaken, gonum/plot doesn't expose primitives to draw ellipses nor arcs of ellipses.
`vg.Path` exposes a `vg.ArcComp` but that's only to draw a circular arc.

one can of course draw an ellipse with a combination of `vg.CurveComp` (quad or cubic ones), according to that paper:
`Drawing an elliptical arc using polylines, quadratic or cubic Bezier curves", L. Maisonobe`

An electronic version may be found at:
- http://spaceroots.org/documents/ellipse/elliptical-arc.pdf

I've already provided such an implementation for Gio.
I could probably provide one for gonum/plot as well.

I'd propose to put it into `vg/draw`:
```go
package draw

// EArc adds an elliptical arc to the path. The implied ellipse is defined
// by its focus points f1 and f2.
// The arc starts in the current point and ends angle radians along the ellipse boundary.
// The sign of angle determines the direction; positive being counter-clockwise,
// negative clockwise.
// If path is nil, a new path is created.
func EArc(path *vg.Path, f1, f2 vg.Point, angle float64) { ... }
```

WDYT?