Explicit category order

[cspotcode]

When you have an "Advanced" command category, you often want that category to appear last in --help. However, it will be alphabetized first.

It would be nice to offer a way to explicitly order command categories and, when #35 is implemented, option categories.

For command categories, perhaps cli.orderCategories([array of category names]) is sufficient.

For option categories, we could accept a similar array on the Usage object, or we could accept an option new Cli({alphabetizeOptions: false}) and document categories in the order they are declared on a command, similar to how positionals are ordered based on declaration order.

[cspotcode]

Thinking out loud, the orderCategories option feels more obvious and straightforward. The array can be declared once, then imported anywhere it is needed.

export const categoryGeneral = 'Options';
export const categoryAdvanced = 'Advanced';
export const categories = [ categoryGeneral, categoryAdvanced ];

import {Categories} from './core';
class Foo extends Command {
  static usage: Usage = {
    categories // name it "categories" instead of "orderCategories" to be terser?
  }
}