top level rails component should automatically pass view helper values

For example
```haml
= mount_component 'Header', controller_name: controller_name, action_name: action_name, flash: flash.as_json
= yield
```
is pretty verbose
instead, just have the top_level_rails component introspect the component being mounted, and
1. if the param is not being provided and
2. if that there is a view_helper with that name, then send the value of the view_helper.

You still have to declare the params in the Component, that is not too bad. Saying it 4 times to access it once, is toooooo much.

Note that is will need #219 to work nicely (i.e. flash.as_json has to be automagic)