Enhancement: remove the CommandInstance construct

[waldekmastykarz]

Related to #1737

After removing dependency on Vorpal, we'll be able to further refactor the command code and remove the CommandInstance construct. It will help us simplify code and tests and remove obsolete code.

If I understood it correctly, Vorpal requires CommandInstance to store information about the current command in its execution context. Since out setup is easier and the context doesn't span beyond the execution of the single command, we can access all the necessary information from the command itself without having to copy it to the context.

I'll pick up this refactoring, after we closed #1737.

[waldekmastykarz]

On a related note, we can also simplify validation. Rather than returning a function or undefined, we can implement validation directly in the validate method and by default have the base Command class return true which means that validation passed. I can include this change in the same refactoring.