Group switch on list

[atoghill]

This PR addresses the first half of #93.

There is now a --group option on the list command that allows the user to list assets within the specified group(s).

[DavidMarchant]

LGTM! thanks
I think having List inherit from MultiNodeCommand isn't the cleanest thing ever (as it isn't reaaally a command which you specify multiple nodes for) but I can absolutely see why you've done it & I can't really think of a nicer way to split up the code base at present so I'll merge it through

[atoghill]

Yeah I wasn't the biggest fan of it but when I initially extracted the main method used to find the assets within a group I kept finding dependencies on other methods. So I opted for the safest change and seeing as it isn't a user facing problem didn't think too much of it 😆