Provide hook for aliases/macros.

[BeekeeperAlex]

Some projects that implement shotgun will want to provide a way to alias commands. I don't want to support aliasing directly in shotgun because it moves too far from the core feature set and makes too many assumptions about the implementing project.

The proper solution would be to come up with some sort of hook into the command parsing logic so that the implementing project can tap into it and make changes. Then the implementing project could do things like pull a list of aliases out of its database and transform them into proper shotgun commands.