Add string interpolation [incomplete]

[Bellisario]

Closes #1.

This is not actually working due problems passing the AST to the parser.

[Bellisario]

@coderaiser, I'm unable to pass baked AST to the parser, the rest of the code is working correctly with only a limitation:

• there isn't a way to "prevent" sting interpolation parsing yet (ex. by adding "\" before string interpolation itself)

[coderaiser]

This is very good!

The thing that is missing is creating the node with this.startNode(), and finishing the node with this.finishNode(), this is how acorn works, it needs it to append the node correctly.

[coderaiser]

We need more tests to make sure that other literals works good, we can proceed in next PR's :).