Named subject

I've got some specs that use the "named subject" syntax

```ruby
subject(:name) { Foo.new }
```

opal-rspec doesn't like this syntax, though. This pull request adds some failing specs for that syntax. I'd love some guidance about how to make the spec pass.

```
$ bundle exec rake

Failures:


  1) named subject should be the subject
     NotImplementedError:
       `super` in named subjects is not supported


  2) named subject should be the named subject
     NotImplementedError:
       `super` in named subjects is not supported

61 examples, 2 failures (time taken: 1.105)
```