Configurability

[bminixhofer]

There should be options which the user can pass:

• upon initializing the Tokenizer / Rules with an extra method (Rust) or keyword arguments (Python).
• with a method set_options at any time afterwards.

This is fairly straightforward to implement. Currently I think the only use case for options is a whitelist once spellchecking (#2) is implemented.

[bminixhofer]

This would be easy to implement now, but is not necessary anymore because:

• The Rules will not get any options to allow collecting into Rules from an iterator over Rule and there are no options that really make sense on the set.
• The Tokenizer will be split up into parts (Modularizing the crate #50), each of which will get options if appropriate and needed.

In summary, with the structure as it is now, options are not needed, but once (if) they are needed they'll be easy to implement.