Add Template method to return template extension

Thanks to Ryan McGrath for the suggestion and initial implementation.

askama/src/lib.rs

@@ -337,6 +337,8 @@ pub trait Template {
         self.render_into(&mut buf)?;
         Ok(buf)
     }
+    /// Helper method to inspect the template's extension
+    fn extension(&self) -> Option<&str>;
 }
 
 pub use askama_derive::*;

askama_derive/src/generator.rs

@@ -117,6 +117,11 @@ impl<'a> Generator<'a> {
         self.flush_ws(WS(false, false));
         self.writeln("Ok(())");
         self.writeln("}");
+
+        self.writeln("fn extension(&self) -> Option<&str> {");
+        self.writeln(&format!("{:?}", self.input.path.extension().map(|s| s.to_str().unwrap())));
+        self.writeln("}");
+
         self.writeln("}");
     }
 

testing/tests/simple.rs

@@ -27,6 +27,7 @@ fn test_variables() {
          Iñtërnâtiônàlizætiøn is important\n\
          in vars too: Iñtërnâtiônàlizætiøn"
     );
+    assert_eq!(s.extension(), Some("html"));
 }
 
 #[derive(Template)]