[Feature Request] add name:'.....' to objects

when creating an object the name always defaults to "object _n_" and allocates _n_ sequentially.

it would be a handy option to be able to override the default name with eg:

```javascript
slide.addShape(
  pptx.ShapeType.hexagon,
  {shape_name:'myHex01', x:0.33, y: 4.51, w: 1.6, h: 1.4, rotate: 270, fill:{ type:'solid', color:'DE1D16' }}
)
```