function registerShape(name, ShapeClass) {
  _customShapeMap[name] = ShapeClass;
}
Register a user defined shape.
The shape class can be fetched by `getShapeClass`
This method will overwrite the registered shapes, including
the registered built-in shapes, if using the same `name`.
The shape can be used in `custom series` and
`graphic component` by declaring `{type: name}`.

@param name
@param ShapeClass Can be generated by `extendShape`.