Similar to #56

People shouldn't specify options that conflict with ours. But if they still do, it would be nice to know whether their parsing conflicts with ours. So I propose to add two tests and document their results in public API:

• my-program --no-input when the custom parser specifies --no-input as well (ideally this should be checked by Tests for CLI #13)
• my-program --no-input my-command --no-input is confusing but ideally should be parsed and return different results for different --no-input. If not, having a test is still good and documenting this behaviour is nice.