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.

Written by the indexing model from the issue text.