Fix issues 190, 194

* Fix handling of empty `options{}` block in grammar (fixes #194)
* Add unit test to verify empty `tokens{}` block is allowed in grammar
* Add error 149: `INVALID_LEXER_COMMAND` (fixes #190)
* Add error 150: `MISSING_LEXER_COMMAND_ARGUMENT`
* Add error 151: `UNWANTED_LEXER_COMMAND_ARGUMENT`