Add continue statement.

tests/while_loop.rs

@@ -6,8 +6,18 @@ fn test_while() -> Result<(), EvalAltResult> {
 
     assert_eq!(
         engine.eval::<INT>(
-            "let x = 0; while x < 10 { x = x + 1; if x > 5 { \
-             break } } x",
+            r"
+                let x = 0;
+
+                while x < 10 {
+                    x = x + 1;
+                    if x > 5 { break; }
+                    if x > 3 { continue; }
+                    x = x + 3;
+                }
+                
+                x
+        ",
         )?,
         6
     );