Make it easier to block on parsing

[xenomachina]

To force parsing, one currently needs to do something like:

    val parser = ArgParser(args)
    val parsedArgs = ParsedArgs(parser)
    parser.force()
    // now you can use parsedArgs

It would be nice if the intermediate ArgParser didn't need to be named, so this could become a one-liner. Perhaps one could write something like:

    val parsedArgs = ArgParser(args).force(ParsedArgs)

This could be done by adding an optional parameter to force, or by creating a new method, hopefully with a better name.