add support for "aria-label" attributes

[davelandry]

For accessibility, it would be great to be able to set the aria-label attribute on each shape type, presumably through an ariaLabel method.

[davelandry]

@rbaheti some more info:

• the attribute should be assigned here: https://github.com/d3plus/d3plus-shape/blob/master/src/Shape/Shape.js#L515
• the default value should be defined in the constructor: https://github.com/d3plus/d3plus-shape/blob/master/src/Shape/Shape.js#L47
• the user-accessible method to change the value will need to be hooked up here: https://github.com/d3plus/d3plus-shape/blob/master/src/Shape/Shape.js#L622