support variables in link contracts

[peacekeeper]

Even though this isn't fully specified by the XDI TC yet, we anticipate that It should be valid to use variables when expressing link contract permissions, e.g.:

(+!:uuid:0000/+!:uuid:8888)$do/$set/{()}

This example would allow $set operations on XDI peer roots (and their subgraphs), but not on other context nodes.