Adds hook scope aliases `example` and `context`

This allows for nicer, more sensible naming in the context of `RSpec.configure` blocks:

```
RSpec.configure do |config|
  # config.before(:all) do     # ... confusing :(
  config.before(:context) do   # sensible!     :D
    # do something
  end
end
```

See #297 for discussion.