Fix long option followed by single dash

[shadowspawn]

A single dash is used with some utilities to represent stdin/stdout, as an argument or as an option value. Minimist was allowing a single dash as a space-separated value for a short option but not for a long option.

Fixes #15

This PR uses the same pattern for checking a potential long-option value as already being used for checking the short-option value. In particular, this means a single dash is now accepted as an option value when used like:

util --foo -

[ljharb]

This is great!

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.75%. Comparing base (ba92fe6) to head (9ec4d27).
Report is 44 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #17   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files           1        1           
  Lines         161      161           
  Branches       71       71           
=======================================
  Hits          159      159           
  Misses          2        2           

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