Configurable view helper

[rmosolgo]

This is my suggestion for a stateful view helper.

• The react_component helper is actually delegated to an internal class, the "implementation class"
• The implementation class is instantiated once per request
• The implementation class has #setup(env) and #teardown(env) methods, in case you're trying to manage state inside the ExecJS environment

This adds an extension point to react-rails: if you want more functionality out of the react_component helper, you can provide a new implementation class.