Target apt updates

[paul-krohn]

Allow targeting apt-get update to only update the sources in a specified file; avoids delays and spurious failures from sources not related to your deployment.

[paul-krohn]

I wasn't sure if I should go ahead and bump the version number; let me know.

[codecov-io]

Codecov Report

Merging #6 into master will increase coverage by 0.44%.
The diff coverage is 96.66%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   94.83%   95.27%   +0.44%     
==========================================
  Files           2        2              
  Lines         213      233      +20     
==========================================
+ Hits          202      222      +20     
  Misses         11       11

Impacted Files	Coverage Δ
tests/test_apt.py	97.69% <100%> (+0.19%)	⬆️
fabric_package_management/apt.py	92.23% <94.44%> (+0.83%)	⬆️

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[andrewsomething]

@paul-krohn Thanks for these additions! At first blush, this looks great. I'll take a closer look this week. Just wanted to let you know I'm not ignoring this.

[andrewsomething]

Thanks again! I'll cut a new release soon.