Add environment variables handeling to static image compositor

[nedelceo]

Same as #1239, which was mistakenly closed by me. Sorry for that.
This PR allows usage of environment variables in StaticImageCompositor.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[mraspaud]

LGTM, thanks for you contribution!

[coveralls]

Coverage increased (+0.004%) to 90.027% when pulling da062c3 on nedelceo:Add_environment_variables_handeling_to_StaticImageCompositor into 4b92a3c on pytroll:master.

[codecov]

Codecov Report

Merging #1241 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1241   +/-   ##
=======================================
  Coverage   90.02%   90.02%           
=======================================
  Files         218      218           
  Lines       31259    31263    +4     
=======================================
+ Hits        28141    28145    +4     
  Misses       3118     3118           

Impacted Files	Coverage Δ
satpy/composites/__init__.py	80.70% <100.00%> (ø)
satpy/tests/test_composites.py	99.88% <100.00%> (+<0.01%)	⬆️

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[mraspaud]

@nedelceo one thing we missed I think: did you add you name to authors.md?

[djhoese]

@nedelceo For future reference, you should be able to reopen a closed PR and you can add new commits to an existing PR to make updates to it. Now that this is merged though, you'll need to make a separate PR to add yourself to the AUTHORS.md file.

Edit: Nevermind, it seems you already did the authors.md stuff. Thanks.