Prevent full stack unwinding at misspelled token

Prevent full stack unwinding at misspelled token.

Once a token is reached that is unknown or misspelled the stack will start unwinding to the parents to see if the token is allowed there. See #3 (closed)

We can prevent this by using get_allowed_tokens() to build a list of all tokens that are allowed on higher levels. If the token IS NOT in the list it will mark it as unknown_token and just continue on the current level. If the token IS in the list it will start unwinding the stack and will be accepted at some point.

This might be added later to prevent complicating the code to much at the early stages.

Edit: This is currently handled with an unchecked_code error message. (issue #21 (closed))