Allow extraction of list items

OAC can be used to get a key from metadata by supplying EG '--key A'.
You can also extract an item from a dict, EG '--key A.B'.
However, if A is a list, there's no way to get an item out of the list.

Extend the expression language (if that's not too grand a term) to
support this. EG '--key A.0' gets the first item out of the list A.
'--key A.1.B' gets B out of the second item of list A (which must be a
dict).

Change-Id: I406cbd2d974544ed5948e137359629184d8ee156
Merge "Allow extraction of list items"