Support 'Stream<String>' return types for commands

[dblevins]

For example:

    @Command
    public Stream<String> listImports(final Dir dir) {
        return dir.searchJava()
                .map(Java::parse)
                .map(Java::getImports)
                .flatMap(Collection::stream);
    }

The contract would be each line is printed as a single line. The newline would be added automatically and need not be present in the individual string values.