run: forward SIGHUP to subprocess

[blueyed]

This is necessary to quit Vim in case input/output is closed.

Ref: AndrewRadev/vimrunner#51 (comment)

[codecov]

Codecov Report

Merging #48 into master will decrease coverage by 0.01%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   97.36%   97.35%   -0.02%     
==========================================
  Files          15       15              
  Lines        1520     1548      +28     
  Branches      165      165              
==========================================
+ Hits         1480     1507      +27     
- Misses         27       28       +1     
  Partials       13       13

Flag	Coverage Δ
#py27	97.09% <97.05%> (-0.02%)	⬇️
#py34	95.73% <97.05%> (+0.01%)	⬆️
#py35	96.57% <97.05%> (-0.01%)	⬇️
#py36	96.57% <97.05%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
tests/test_cli.py	100% <100%> (ø)	⬆️
covimerage/cli.py	94.11% <90.9%> (-0.33%)	⬇️

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