Lazy-loaded commands

[seeruk]

Extracted from work on an internal tool at Icelolly, this would be useful for larger applications with sprawling dependencies.

// We could make a function for adding factories for commands, to enable lazy-loading of commands,
// meaning their dependencies wouldn't be created if they weren't going to be used. Added like:
//
// app.AddCommandFactory("apply", resolver.ResolveApplyCommand)
//
// On the resolver, we'd have our factory function.
//
// func (r *Resolver) ResolveApplyCommand() *console.Command {
//     // ...
// }
//
// For sub-commands, we'd need to just do the same, but the factory functions would be passed as
// dependencies in the resolver. Something like:
//
// func (r *Resolver) ResolveContainerCommand() *console.Command {
//     return container.NewCommand(
//         r.ResolveContainerListCommand,
//         r.ResolveContainerInspectCommand,
//         // ...
//     )
// }
//
// Note that the factory function itself is passed in, the resolver is not being invoked at this
// point. This stops us from passing the resolver around, meaning that testing should still be easy
// enough to accomplish. We could just replace the sub-commands in tests with a no-op factory.