**Rationale:** Realized that #10 did not include any way to register commands without going through the IntoCommandDefinition trait. **Changes:** - Adds a method to register a raw command handler Reviewed-on: #11 Co-authored-by: Sergey Savelyev <sergeysav.nn@gmail.com> Co-committed-by: Sergey Savelyev <sergeysav.nn@gmail.com>