Unify @Option and @Parameters annotations

[remkop]

Options on how to do this:

• Combine into single annotation "Parameter"
• Shared super-interface ("Parameter"), Option and PositionalParameter

+----------+         +---------------+        +---------------------+
| Option   | ----->  |  Parameters   | <----- | PositionalParameter |
+----------+         +---------------+        +---------------------+
| names    |         | description   |        | position            |
| required |         | arity         |        +---------------------+
| help     |         | paramLabel    |         
+----------+         | type/converter|         
                     | hidden        |         
                     +---------------+         

[remkop]

Annotations don't support inheritance.

I considered following JCommander's design of having a single annotation to denote both options and positional parameters, but I'm not convinced it's a good idea. (There's an experimental branch where this idea is implemented.) Having a single annotation seems to simplify things but it turns out that it just moves the complexity. Options are different from positional parameters: they have one or more names and may take a parameter. Positional parameters have an index. Trying to model this in a single annotation means a lot of conditional logic in picoCLI, and for users it can be confusing: should I provide both a name and an index? What if I specify neither?

Separate annotations seems cleaner. For example, it allows us to make the @option names attribute mandatory, enforced by the compiler, which is very nice. I think the explicit separation makes the concepts easier to grasp for users, so this is a better design.