input template generator

[mlin]

Adds a new subcommand miniwdl input-template to generate a JSON skeleton for the inputs required to run a given WDL. Once filled out, the JSON can then be used with miniwdl run ... -i INPUTS.json.

#553 & builds on work started by @xquek in #639

[mlin]

@xquek I had some time this past weekend to build on your #639 -- thanks again for getting it started! Key points:

• I simplified it to templating only the required inputs for now. I need to think about what to put in the template for optional inputs that have defaults, e.g. Int read_length = 150. If it's a constant like that then obviously copy it in, but what if it's some WDL expression with string interpolations etc.? I want to come back to this later but I think the tool is pretty useful even if it's only considering the required inputs.
• Kept the recursive logic next to the entry point in CLI.py for now. Ultimately I see it moving into the package API as you had it, but keeping it experimental right now while we gain some experience with it and think through the above issue.
• General polishing & docs

[coveralls]

Pull Request Test Coverage Report for Build 5793365510

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.03%) to 95.17%

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Files with Coverage Reduction	New Missed Lines	%
WDL/Lint.py	1	98.4%
WDL/runtime/task.py	1	95.96%

Totals
Change from base Build 5643052456:	-0.03%
Covered Lines:	7350
Relevant Lines:	7723

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💛 - Coveralls