Ensure editor and the file to edit can have spaces in them

[kotfu]

The unit tests monkey patch the os.system call so its hard to verify this actually works.

I tested manually on macOS with python3.6 and spaces in both editor and the filename supplied to the edit command. Both work fine.

[codecov]

Codecov Report

Merging #212 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #212   +/-   ##
=======================================
  Coverage   96.74%   96.74%           
=======================================
  Files           1        1           
  Lines        1166     1166           
=======================================
  Hits         1128     1128           
  Misses         38       38

Impacted Files	Coverage Δ
cmd2.py	96.74% <100%> (ø)	⬆️

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