Allow colormaps to be saved with geotiff writer

[katherinekolman]

This makes it so users can keep the palettized version of the image and apply a colormap to it. There's a related pull request in trollimage.

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[coveralls]

Coverage increased (+0.2%) to 81.575% when pulling 77c3d14 on katherinekolman:colormap into d13b670 on pytroll:master.

[codecov]

Codecov Report

Merging #580 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #580      +/-   ##
==========================================
+ Coverage   81.41%   81.57%   +0.16%     
==========================================
  Files         154      154              
  Lines       22447    22583     +136     
==========================================
+ Hits        18276    18423     +147     
+ Misses       4171     4160      -11

Impacted Files	Coverage Δ
satpy/writers/geotiff.py	43.54% <100%> (+1.88%)	⬆️
satpy/tests/writer_tests/test_geotiff.py	93.5% <100%> (+0.85%)	⬆️
satpy/enhancements/__init__.py	85.33% <100%> (+8%)	⬆️
.../tests/reader_tests/test_viirs_edr_active_fires.py	97.16% <0%> (+0.98%)	⬆️
satpy/readers/viirs_edr_active_fires.py	93.61% <0%> (+3.61%)	⬆️

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[djhoese]

@mraspaud if I fix the conflict in this, do you think we could/should merge this? Or should we wait for me or Kat to add documentation on how it should be used?

[mraspaud]

Just some documentation in the docstring would be enough for me