[WDL 2.0] task hints

[mlin]

Provisionally implements https://github.com/openwdl/wdl/pull/315

• Add task hints section
  • Modeled on meta sections, with nested literals but no expressions
  • Unused for now
• Allow JSON/CLI input overrides for runtime values
• Alias runtime.container to runtime.docker, and accept arrays (unused for now)

[coveralls]

Pull Request Test Coverage Report for Build 483887157

• 4 of 4 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 96.577%

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Totals
Change from base Build 483876129:	0.02%
Covered Lines:	6574
Relevant Lines:	6807

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💛 - Coveralls