Add command line arguments to all examples. Were already on some.

[freemansoft]

• Add command line arguments to all shelf example programs.

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• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Details

• One of the examples already had command line arguments. I rationalized all the example programs taking the same arguments and adding one in one case.
• Commandline arguments are cool. They clutter the examples but make them more flexible.
• Did not add --help
• Not fixed: Some of the example programs are example.dart. Others are main.dart