Allow some field values to refer to other field values by name

[jlebon]

For example, it's common for the context to be the same as the distro used. In that case, it would be nice if one could just do something like:

context: $distro

Which gets properly inherited and such.

Another potential use:

env:
  TEST: mytest

context: "$distro | $TEST"

Of course, this would require careful examination of string origins and sanitization to make sure there's no nastiness possible.