Add support for abstract and extends

[labra]

We are introducing two new language features to ShEx that would fit quite nice in an UML visualization:

• abstract declares shape as abstract
• extends declares that a shape is a sub-shape of another one.

The syntax can be seen in the following example: http://rdfshape.weso.es/link/16064718191

[ExarcaFidalgo]

Abstract seems like a simple one. I'm somewhat worried about the extends, though... I must check a couple things about our current use of inheritance.

[ExarcaFidalgo]

By the way, we are dependent on shex.js NPM package to parse the input. No updates have taken place since we started this project, so these features do not seem to be supported yet...

[labra]

Ops, maybe tha's true. At this moment extends and abstract are indeed implemented in shex.js, but they are implemented in the extends branch.

I will add a issue to shex.js to ask Eric to publish an npm package supporting extendsand abstract. One problem is that this is still work-in-progress and has not been approved by the ShEx community group...