Add feature to disable symbols.

doc/src/engine/disable.md

@@ -0,0 +1,28 @@
+Disable Certain Keywords and/or Operators
+========================================
+
+{{#include ../links.md}}
+
+For certain embedded usage, it is sometimes necessary to restrict the language to a strict subset of Rhai
+to prevent usage of certain language features.
+
+Rhai supports surgically disabling a keyword or operator via the `Engine::disable_symbol` method.
+
+```rust
+use rhai::Engine;
+
+let mut engine = Engine::new();
+
+engine.disable_symbol("if");    // disable the 'if' keyword
+engine.disable_symbol("+=");    // disable the '+=' operator
+
+// The following all return parse errors.
+
+engine.compile("let x = if true { 42 } else { 0 };")?;
+//                        ^ missing ';' after statement end
+//                      ^ 'if' is parsed as a variable name
+
+engine.compile("let x = 40 + 2; x += 1;")?;
+//                                ^ '+=' is not recognized as an operator
+//                         ^ other operators are not affected
+```