Allow no value

I'm trying to implement a string option that can have three states:
1. Not supplied
2. No value
3. Empty or value

I want to do the following:
`MyApp.exe` writes unformatted output to standard output
`MyApp.exe --json` writes JSON formatted output to standard output
`MyApp.exe --json file.json` writes JSON formatted output to `file.json`

Is this currently possible?

The issue names no files or tests. Start by checking the command-line option parsing entry points and whether an option can distinguish omission, a supplied option with no value, and an empty or explicit value. Verify the three command forms in the issue and define tests for each expected output and destination.

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