Add support for decorators

[acairns]

Use Cases:

• I need to validate a command before executing it. I want this to happen before a handler is resolved.
• I need to log the handling of all my commands. I want this added as a decorator straight after a command has been handled.

Implementation should allow for decorators to happen before and/or after the handling of a command. Also, each command should be allowed it's own configuration of decorators, not the bus itself.