Add quality attribute for audio questions

[seadowg]

This adds support for the quality attribute detailed here.

Why is this the best possible solution? Were any other approaches considered?

Didn't seem like there were a lot of different ways to go but definitely keen to hear criticism as I'm pretty new to the codebase.

What are the regression risks?

Only audio questions have been changed so that's the only area where something could go wrong (I'd imagine).

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Yes but that's been planned separately!

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[codecov-io]

Codecov Report

Merging #483 (880e916) into master (85582ef) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
+ Coverage   83.63%   83.67%   +0.04%     
==========================================
  Files          25       25              
  Lines        3641     3651      +10     
  Branches      846      849       +3     
==========================================
+ Hits         3045     3055      +10     
  Misses        452      452              
  Partials      144      144              

Impacted Files	Coverage Δ
pyxform/xls2json.py	78.30% <100.00%> (+0.33%)	⬆️

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[lognaturel]

Good to see a README section about tests.

[MartijnR]

Looks good to me 👍