support variables in link contracts

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.
