- don't output duplicate entries based on unique field parameter
  - introduce unique field parameter to specify TSV per record unique
field. If an output record already has the field's value, then don't
output that record into the aggregate JSON. 
  - for back compatibility, if the unique parameter isn't specific then
the program will simply behave as if there are no unique fields.
  - if unique is specified and the field does not exist in the TSV
record, then the document is simply added to the aggregate JSON.
Merge branch 'master' of https://github.com/chrismattmann/etllib
Merge branch 'master' of https://github.com/chrismattmann/etllib