Allow override core operators

[cnogueira]

Resolves #455.

Added allowOverrideCoreOperators flag to PebbleEngine.Builder. It defaults to false to preserve backwards compatibility.

Example usage:

    PebbleEngine pebble = new PebbleEngine.Builder()
        .loader(new StringLoader())
        .extension(new TestExtension())
        .allowOverrideCoreOperators(true)
        .build();

If enabled, it will apply all operators defined in provided extension class.
Otherwise it will only apply those operators that are not already defined in CoreExtension.class operators.