ensure the current task is stopped before exit

[jesseduffield]

fixes #147 (kinda)

For some reason we have child processes that are not being killed when lazydocker exits. To fix this, I'm making it so that before closing, lazydocker kills its task manager, which in turn kills the current task if there is any (e.g. docker logs <container id>). If the child process isn't killed in three seconds, it prints an error to stdout.

I have no idea why the child processes are not being killed when the parent is stopped in the first place. I considered maybe their group id is different, given we assign a group id to some subprocesses like docker-compose logs, but it seems that's not the case with docker logs. I am open to suggestions :)

[codecov-io]

Codecov Report

Merging #148 into master will decrease coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   26.44%   26.12%   -0.33%     
==========================================
  Files          13       13              
  Lines        1138     1129       -9     
==========================================
- Hits          301      295       -6     
  Misses        821      821              
+ Partials       16       13       -3

Impacted Files	Coverage Δ
pkg/config/app_config.go	82.38% <0%> (-3.78%)	⬇️
pkg/commands/container.go	0% <0%> (ø)	⬆️
pkg/commands/docker.go	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a14e640...3f28a97. Read the comment docs.