check for sigint before setting exit code to 1 - Fixes #180

[jnielson94]

What:
Fix #180. Change logic in child process exit listener to the following:

1. Default to the exit code passed in
2. Default to exit code 0 if exit code is null but signal was SIGINT
3. Default to exit code 1 otherwise

How:
See #180 for documentation of how fix was found/decided on.

Checklist:

• Documentation N/A
• Tests
• Ready to be merged
• Added myself to contributors table

I couldn't get the precommit hooks to work, but I ran validate, build, test, and manually used the all-contributors-cli to add myself to that section.

@kentcdodds I didn't feel like this needed any documentation outside of the code, feel free to point me where some could be used if there is a place.

[codecov]

Codecov Report

Merging #181 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #181   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          65     68    +3     
  Branches       16     17    +1     
=====================================
+ Hits           65     68    +3

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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[kentcdodds]

Super! Thank you very much :) Sorry about the trouble with kcd-scripts and all-contributors.