Rework command subsystem

[jvalkeal]

One trouble in #379 when I started to implement it is that while it's easy to make CommandRegistry writable and register new methods based on MethodTarget this process from a user perspective is a bit cumbersome.

Essentially we still need to make custom pojo:

	public static class PojoMethods {

		@ShellMethod
		public String dynamic1() {
			return "dynamic1";
		}

		@ShellMethod
		public String dynamic2(String arg1) {
			return "dynamic2" + arg1;
		}
	}

and have those methods with dummy @ShellMethod for StandardParameterResolver to read parameters.

[jvalkeal]

This turned out to be a relatively major work so it is unrealistic to do it with 2.1.0. Marking 3.0.x for now and can be hopefully back ported to 2.2.x.

[jvalkeal]

Just wanted to give update on this as I've been doing a major refactoring and re-work so there might be a time to push this work into 2.1.x. It's definitely something what can't be added in a point release so it'd be really beneficial to get these changes in 2.1.x.