Fixes jarro2783#240: Multiple long option names / aliases

* We now use a vector of long option names instead of a single name
* When specifying an option, you can provide multiple names separated by commas, at most one of which may have a length of 1 (not necessarily the first specified name). The length-1 name is the single-hyphen switch (the "short name").
* Hashing uses the first long name
* Option help currently only uses the first long name. Not sure if that should change.
* Almost no tests for this functionality yet
* Using `std::token_regex_iterator` to tokenize for option-splitting

Author: eyalroz