Add feature to disable symbols.

tests/tokens.rs

@@ -0,0 +1,20 @@
+use rhai::{Engine, ParseErrorType};
+
+#[test]
+fn test_tokens_disabled() {
+    let mut engine = Engine::new();
+
+    engine.disable_symbol("if"); // disable the 'if' keyword
+
+    assert!(matches!(
+        *engine.compile("let x = if true { 42 } else { 0 };").expect_err("should error").0,
+        ParseErrorType::MissingToken(ref token, _) if token == ";"
+    ));
+
+    engine.disable_symbol("+="); // disable the '+=' operator
+
+    assert!(matches!(
+        *engine.compile("let x = 40 + 2; x += 1;").expect_err("should error").0,
+        ParseErrorType::BadInput(ref s) if s == "Unexpected '+='"
+    ));
+}