add background execution of git-wip

[bartman]

add background config option. if set will run git-wip in the background to lower the impact on neovim's responsiveness.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.28%. Comparing base (f62b78c) to head (bb1bcec).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
  Coverage   71.28%   71.28%           
=======================================
  Files          17       17           
  Lines         700      700           
  Branches      114      116    +2     
=======================================
  Hits          499      499           
  Misses        201      201           

Flag	Coverage Δ
unittests	71.28% <ø> (ø)

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