Add options for creating geotiff overviews

[djhoese]

Adds the additional keyword arguments necessary to take advantage of trollimage's new overviews functionality. See pytroll/trollimage#67.

The basic overviews functionality has existed for a while, but this keyword argument was never actually passed to trollimage. Not sure who was using overviews, but I don't think it ever worked.

Given this is just kwargs, do I need to add tests?

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

[coveralls]

Coverage decreased (-0.003%) to 89.615% when pulling ae4aea5 on djhoese:feature-geotiff-overviews into ef24f29 on pytroll:master.

[codecov]

Codecov Report

Merging #1183 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1183      +/-   ##
==========================================
- Coverage   89.61%   89.61%   -0.01%     
==========================================
  Files         200      200              
  Lines       29522    29523       +1     
==========================================
  Hits        26457    26457              
- Misses       3065     3066       +1     

Impacted Files	Coverage Δ
satpy/writers/geotiff.py	90.74% <ø> (ø)
satpy/scene.py	90.01% <0.00%> (-0.18%)	⬇️
satpy/tests/reader_tests/test_fci_l1c_fdhsi.py	100.00% <0.00%> (ø)

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[mraspaud]

LGTM. I suppose it doesn't make sense to add a test?

[djhoese]

I'm going to say no on the tests. Merging now...