[MRG] Fix --output-unassigned to output abundances if available.

[ctb]

Fixes #705.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #726 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #726      +/-   ##
==========================================
+ Coverage   89.27%   89.28%   +<.01%     
==========================================
  Files          29       29              
  Lines        4374     4378       +4     
  Branches       45       45              
==========================================
+ Hits         3905     3909       +4     
  Misses        467      467              
  Partials        2        2

Impacted Files	Coverage Δ
sourmash/commands.py	88.77% <100%> (+0.05%)	⬆️

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[ctb]

Ready for review & merge @luizirber @taylorreiter @bluegenes @olgabot