This should also speeds up the parser (except maybe for short options).
Multi-codepoint characters still can't be used as short options, but people shouldn't non-ASCII options nyway.
Closes#664
Prior to this change, values were always delimited by default. This was causing issues with code
where the arg had a single value, and contained valid commas and shouldn't be delimited. This
commit changes the rules slightly so that values are not delimited by default, *unless* one of the
methods which implies multiple values was used (max_values, value_names, etc.).
This means single value args should *not* be delimited by default. If one wishes to use the old
way, they can add `Arg::use_delimiter(true)` to such code.
Closes#655
