HPCC-2566 Allow attribute values to be specified on the command line

Also allow ^ as a synonym for the root scope.

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
This should really be in a PR and Jira issue all of its own. @ghalliday  I remember you and I conversing about this around the time when you added '^' into that grammar in HPCC-10150. I think I wanted to add this into the new grammar in the new parser but there was a reason that perhaps you felt otherwise, I can't quite remember. I'm looking through logs now and some tests that I still have lying around. Also checking on whether any doc changes are needed.