Enable "dx download <job_id>:<output_field>".

Summary:
Can download output file created by job by specifying job id (e.g.
job-xxxx) and output field (e.g. reads) of output file according to
following: "dx download job-xxxx:reads".

Test Plan: DXTEST_RUN_JOBS=1 ./test_dxclient.py TestDXClientUploadDownload.test_dx_download_by_job_id_and_output_field

Reviewers: psung

Reviewed By: psung

Subscribers: psung

Differential Revision: https://phabricator.dnanexus.com/D129