Add support for Renderer constructors in Erb handler

[bradgessler]

The Erb handler currently doesn't support arguments in the Renderer constructor due to the way the ActionView renderer works at https://github.com//sitepress/markdown-rails/blob/main/lib/markdown-rails/handler/erb.rb#L8

I know how to get this working, but I took this shortcut to ship an alpha.

Add a way to reference the Handler from the Rails renderer.