Improving help message formatting.

Test Plan: None

Reviewers: klai

Reviewed By: klai

Differential Revision: https://phabricator.dnanexus.com/D126
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
Changelog and version updates for v0.121.0
Cleanups to "dx find projects"

* Remove blank line at end of (non-brief) output
* Don't buffer entire result set in memory
* Don't update cached_project_names since it never gets read
* Conform to PEP 8 whitespace and line length guidelines
* Chip away at the monolithic test test_dx_actions in favor of more
  specific test suites
PTFM-7932 Lower findProjects query page size for "dx select"

The previous page size was 100. Since the user is unlikely to page
through to 100 results in 10s, we only fetch 10 results initially.
PTFM-12607 Improving bash helper variables

Summary:
1. Refactoring of code, so as to acheive sharing, between local and remote execution.
2. Adding several bash variables available to the application: filenames, prefixes, and paths.

Test Plan: additional app for testing.

Reviewers: klai

Reviewed By: klai

Subscribers: psung

Differential Revision: https://phabricator.dnanexus.com/D88
Merge branch 'master' into PTFM-11906-dx-cp-None-clone