Added the signature for passing filters into the render function.

Added the signature for passing filters into the render function but I am not yet using the filters.

src/parser/mod.rs

@@ -9,5 +9,6 @@ pub use node_invoker::Result;
 pub use parser::template;
 pub use parser::Body;
 pub use parser::DustTag;
+pub use parser::Filter;
 pub use parser::Template;
 pub use parser::TemplateElement;