Use a variable to access a hash

[jmks]

I took a shot at fixing #142

Added an identifier to object access (i.e. between [..]). The tricky part is that ends up as a string when reaching Context.get_in/3, so you can't know if you want to access that value or the value a variable points to.

So I tagged the value with :reference (maybe that's not a great name). Then Context.get_in/3 has to resolve what value that variable points to.

I wrote a test showing that a variable pointing to a variable pointing to a variable works but that may "just work" because of how the Assign works. So maybe that test is unnecessary.

That seems to work but I'm not sure if there's a better way to do this.

[edgurgel]

This looks good to me!

I think the only thing that we might want to add in the future is that with strict_variables options we want to warn if a variable is not defined but that can be done as a separate piece of work.

Thank you!