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>
@richardkchapman please check this logic.  
Previously the dash code would have made an option of --allow set optName to ('') - hence the previous test against start