Add extension to pass custom environment variables

[meyerj]

Mimics the -e, --env option of docker run:

       -e, --env=[]
          Set environment variables

       This option allows you to specify arbitrary environment variables that are available for the process that will be launched inside of the container.

Probably shlex.quote should be applied to other extensions that allow the user to pass custom command line arguments, too, for example #46.

[tfoote]

Thanks for adding this. I was thinking of trying to work to avoid people passing arbitrary environment variables through and keep things in extensions. But that's too much overhead for many things.

I added some basic unit tests too.

[codecov]

Codecov Report

Merging #51 into master will increase coverage by 0.66%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
+ Coverage   82.66%   83.33%   +0.66%     
==========================================
  Files           5        5              
  Lines         375      390      +15     
==========================================
+ Hits          310      325      +15     
  Misses         65       65

Impacted Files	Coverage Δ
src/rocker/extensions.py	100% <100%> (ø)	⬆️

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