Fix commands to properly invoke base and extensions

In previous work done during the command refactor the mechanism for overriding
commands changed such that it wasn't invoking commands if they were overriden
by an extension. This, it would appear, broke some stuff. Badly.

This commit fixes this issue by reinstating the way things were done before.
If a base command exists, it is always executed. If an extension also exists
which overrides this command then the base command result is ignored, the
extension command is executed, and the result of that command is returned.