Fix writing options that contain spaces

[odow]

Closes #196

The Cmd wraps things in ' ', but this happens to the entire 'output_file=/path/to/file.txt', and the binary fails to parse that.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.87%. Comparing base (9e5b590) to head (b035c60).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   98.80%   98.87%   +0.06%     
==========================================
  Files           1        1              
  Lines          84       89       +5     
==========================================
+ Hits           83       88       +5     
  Misses          1        1              

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