Add support to render templates directly

[justinperkins]

I realize this is a bit of an edge case, but it'd be really great to be able to render templates outside the normal Rails usage – such as to be able to render a template inline from the controller, but with all the normal instance variables and view helpers available to you just like you'd have if Rails was rendering the template in the view layer.

I am trying to replace some existing Rabl rendering with json_builder, but this is the one thing Rabl does well that json_builder currently does not. See their docs under "Rendering templates directly" for more information.