feat: improve error message on unexpected options

[NickCrews]

Attempt at addressing #5

TODO: ensure we have test cases for each of the examples brought up in that issue:

• --dry-run vs --dryrun
• Usage: cli <arg1> <arg2> & the usage of cli abc currently gives Warning: found unmatched (duplicate?) arguments [Argument(None, '1')], this should be better

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.58%. Comparing base (587f41a) to head (1345d07).

Files with missing lines	Patch %	Lines
docopt/__init__.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master      #77      +/-   ##
===========================================
- Coverage   100.00%   99.58%   -0.42%     
===========================================
  Files            2        2              
  Lines          464      480      +16     
===========================================
+ Hits           464      478      +14     
- Misses           0        2       +2     

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