Beginning of the render_tag function.

Beginning of the render_tag function. At this point I need to figure out how to access elements in the context.

src/parser/mod.rs

@@ -7,5 +7,6 @@ pub use node_invoker::run_node_dust;
 pub use node_invoker::NodeError;
 pub use node_invoker::Result;
 pub use parser::template;
+pub use parser::DustTag;
 pub use parser::Template;
 pub use parser::TemplateElement;