add --env & config option to set values in all task environments

[mlin]

@jgadling building on your much-appreciated #516, I extended it to work just like docker run --env:

• --env X to pass through environment variable X, skipping if X is undefined
• --env X=foo to set X outright
• supply --env multiple times with different variables

Let me know if you have any concerns with this variation on it!

While hacking on this, I remembered how miniwdl passes AWS credentials to its internal S3 downloader: it generates a temporary script file defining the necessary environment variables, then sources that script at the start of a task that needs it. The advantages of this approach are (i) it's concise yet explicit that the task needs this information supplied and (ii) miniwdl is careful never to dump file contents into its log in case they contain sensitive information. Just another concept to consider, however, I also support this passthrough feature.

[coveralls]

Pull Request Test Coverage Report for Build 1062207074

• 15 of 15 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 96.51%

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Files with Coverage Reduction	New Missed Lines	%
WDL/runtime/task.py	1	95.22%

Totals
Change from base Build 1061778500:	0.02%
Covered Lines:	6720
Relevant Lines:	6963

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💛 - Coveralls

[jgadling]

Awesome, I like the extra functionality!