Compile fix for external projects + vibe.d convenience functions.

Hi! I finally got around to try out embd and made some working `render!()`-like functions for use in a vibe.d context. Everything worked like a charm once it compiled. I just needed to move the `createRenderingCode` into `Context` so that it is accessible from within the mixin in external modules. The vibe.d specific functions will only be compiled if "vibe-d" was explicitly added to a project as a dependency.