placeholder within placeholder

[jjfraney-cg]

Please enable nested properties. Or in other words, resolve property values using other property values (and the context).

For example, if following are pulled from spring cloud configuration service:

nestL1 = [ L1 - ${nestL2} - L1 ]
nestL2 = [ L2 - ${nestL3} - L2 ]
nest = top - ${nestL1} - top

and context has:
nestL3 = [ L3 - deepest - L3 ]

Then config.get('nest') gives: "top - [ L1 - [ L2 - [ L3 - deepest - L3 ] - L2 ] - L1 ] - top"