This commit introduces a new `register()` method on the CommandHandler
class to allow registering pre-instantiated subcommands. This means that
subcommands are no longer restricted in terms of instantiation, so they
can have their dependencies injected at creation, rather than having to
resort to Singleton anti-pattern means.
Also refactors the existing internal MobArena command registration to
use the new method to "drink our own champagne" and to reuse the code.
Fixes#675
Andreas Troelsen
903752d23a