Add support for mutual exclusive options

Something like this:

@Command(autoHelp = true, sortOptions = false, groups = {
@OptionGroup(name = "dest", required = true, exclusive = true,
           heading = "Destination: The data can be sent either to a file or to a socket"),
@OptionGroup(name = "other",
           heading = "Other:%n  Some other options")})
class ExclusiveOptionsExample {

  @Option(names = "-f", description = "Target file", group = "dest")
  File file;
  
  @Option(names = "-s", description = "Target socket", group = "dest")
  InetAddress adr;
  
  @Option(names = "-d", description = "Data file", group = "other")
  File data;
}

Usage help:

Usage: <main class> [-hV] [-d=<data>] (-f=<file> | -s=<adr>)
Destination: The data can be sent either to a file or to a socket
  -f= <file>                  Target file
  -s= <adr>                   Target socket
Other:
  Some other options
  -d= <data>                  Data file
  -h, --help                  Show this help message and exit.
  -V, --version               Print version information and exit.
Commands:
  help  Displays help information about the specified command

[remkop]

A first cut of support for ArgGroups has landed in master. This adds support for:

• mutually exclusive options and positional parameters (exclusive = true)
• co-occurring options and positional parameters (exclusive = false: require all elements in the group, or none) as requested in #295
• composite groups
• groups can be required or optional

Example:

 @Command(argGroups = {
         @ArgGroup(name = "EXCL",      exclusive = true,  required = true),
         @ArgGroup(name = "ALL",       exclusive = false, required = true),
         @ArgGroup(name = "COMPOSITE", exclusive = false, required = false,
                   subgroups = {"ALL", "EXCL"}),
 })
 class App {
     @Option(names = "-x", groups = "EXCL") boolean x;
     @Option(names = "-y", groups = "EXCL") boolean y;
     @Option(names = "-a", groups = "ALL") boolean a;
     @Option(names = "-b", groups = "ALL") boolean b;
 }

This defines a composite group with the following synopsis:

 [(-x | -y) (-a -b)]

The composite group consists of the required exclusive group (-x | -y) and the required co-occurring group (-a -b).

Valid user input for this group would be:

• no options at all (because the composite group itself is non-required)
• -x -a -b (in any order)
• -y -a -b (in any order)

Any other combination of -x, -y, -a and -b would give a validation error.

---

Still TODO:

• show group synopsis in usage help message
• show group headings in options list (as requested in #450)
• resource bundle support for group headings in options list (related to #450)
• update annotation processor in picocli-codegen to build a model from the new annotations at compile time
• documentation in user manual and release notes

That said, the majority of the functionality is there. I would appreciate it if people would try this to see if it meets their requirements and to find any places where the API, error messages or anything else should be improved.

[lenkite]

Also: Sometimes its common to have a single option as an alternative with a group of options. Like in the below -u, -p, -c are only required if -P (profile) is not present.

list-nodes -c <controlHost> -u <user> -p <pass>  OR list-nodes -P <profileName>