Add option to use tempfiles for IPC

[evanunderscore]

Supersedes #306, fixes #267, fixes #142.

Adds support for Git Bash (and potentially other shells on Windows) by giving the option to use a file for IPC instead of file descriptors.

Currently this feature is opt-in as described in the README.

[evanunderscore]

@kislyuk Given the lengthy discussion on this so far, I'll leave this to you to approve and merge. If you prefer, I can make this the default on Windows instead of being opt-in.

[codecov-commenter]

Codecov Report

Merging #307 into master will decrease coverage by 0.39%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
- Coverage   82.72%   82.32%   -0.40%     
==========================================
  Files           7        7              
  Lines         741      747       +6     
==========================================
+ Hits          613      615       +2     
- Misses        128      132       +4     

Impacted Files	Coverage Δ
argcomplete/shell_integration.py	100.00% <ø> (ø)
argcomplete/__init__.py	90.35% <33.33%> (-0.96%)	⬇️

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[kislyuk]

Thanks @evanunderscore for sticking with the long development cycle for this.

[evanunderscore]

Thanks for your review!