SSE class for ActionController::Live

I've created a class that will handle sending Server Sent Events (SSE)'s to the browser through the `ActionController::Live` module. Since this module provides the ability to stream data, it makes sense to also be able to send data in the form of an SSE.

This PR provides a way to create SSEs quickly from provided data. Hopefully, this will make it easier for Rails developers to send continuous message streams to the browser since they won't have to write their own SSE helper class.

\cc @spastorino