Add support for option modifiers

The programmer can enable modifiers for options which changes how the cli interprets the value for the option. Inspired by the Java Keytool

For example: -option value would use the value directly for option. -option:env value would treat value as a environment variable to look up for the actual value to supply to option. -option:file value would treat value as a path to a file to read for the value supplied to option

Useful for application wishing to hide sensitive information, such as passwords, from the command line.

[remkop]

To summarize, the password prompting feature is covered by this ticket: #82. Other than that, -option, -option:env and -option:file options are treated as separate options by picocli, which provides the building blocks to build a custom solution as described in my previous comment.

I won’t look further at providing special library support for option modifiers, unless something new comes up.

Still this is a good idea to either write an article about or mention in the user manual after #82 is implemented. (I added this to the list in the wiki: https://github.com/remkop/picocli/wiki/Plans-and-Notes-for-Future-Articles)

[remkop]

I’ve thought about this some more and I was wondering if special support is really needed. One can already do this:

class Args {
    @Option(names = "-option", description = "the option value")
    String option;

    @Option(names = "-option:env", description = "the environment variable to look up for the actual value")
    String optionEnvKey;

    @Option(names = "-option:file", description = "path to the file containing the option value")
    File optionFile;
}

The usage help for such a class looks exactly as one would expect:

Usage: <main class> [-option=<option>] [-option:env=<optionEnvKey>] [-option:
                    file=<optionFile>]
      -option=<option>        the option value
      -option:env=<optionEnvKey>
                              the environment variable to look up for the
                                actual value
      -option:file=<optionFile>
                              path to the file containing the option value

Wouldn’t it be better to leave the next step (calling System.getenv(args.optionEnvKey) or Files.readAllBytes(optionFile.toPath())) to the application?