Allow customization of vimfiler_quick_look_command

[crisidev]

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Why this change is necessary and useful?

Closes #1888

[codecov]

Codecov Report

Merging #1889 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #1889   +/-   ##
=======================================
  Coverage   26.39%   26.39%           
=======================================
  Files         253      253           
  Lines       12592    12592           
=======================================
  Hits         3324     3324           
  Misses       9268     9268