Command namespaces

[AaronMcHale]

I don't have any specific need for this just now, but namespaces for commands could be a useful way of grouping commands if there are a lot.

I'm imagining something like this:

1. A namespace is defined as a sub-directory of the commands directory.
2. Any scripts within the sub-directory have their commands automatically prefixed with the name of the namespace. For instance, if we moved the hello-world script into a examples sub-directory, then the hello-world command would become examples:hello-world.
3. The command list could then show commands grouped by their namespaces, with any scripts in the top-level commands directory being shown without a namespace.
4. Existing scripts in the top-level commands directory would remain unchanged and would work as they do now.
5. If we do add command aliases, see Support for command aliases #6, aliases would not be namespaced, so in the above example, if the examples:hello-world command had the alias hello, you could run the command hello or examples:hello-world.