Improve customisation in multiscene creation

[gerritholl]

When creating a MultiScene using MultiScene.from_files, allow the user
to pass additional keyword arguments to the Scene creation with the
scene_kwargs keyword argument.

• Closes Customes Scene creation from MultiScene.from_files #1399
• Tests added
• Passes flake8 satpy
• Fully documented

[codecov]

Codecov Report

Merging #1400 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1400   +/-   ##
=======================================
  Coverage   90.56%   90.56%           
=======================================
  Files         228      228           
  Lines       33406    33408    +2     
=======================================
+ Hits        30254    30256    +2     
  Misses       3152     3152           

Impacted Files	Coverage Δ
satpy/tests/test_multiscene.py	99.42% <ø> (ø)
satpy/multiscene.py	90.68% <100.00%> (+0.05%)	⬆️

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[coveralls]

Coverage decreased (-0.03%) to 90.535% when pulling ce74650 on gerritholl:multiscene-custom-scene-args into 12917ab on pytroll:master.

[gerritholl]

Can I do anything about the erroneous coveralls report, or is there anything else to do on this PR?

[djhoese]

I've mentioned this before on slack, but I'm not a huge fan of where all these kwargs changes are going, but I'm not sure we have a choice right now either. As is, this PR does what it is meant to do and looks good to me.