add support for packbits compressed images

[geospatial-jeff]

• Adds support for packbits compressed images

Ref #53

[codecov-commenter]

Codecov Report

Merging #54 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
+ Coverage   95.52%   95.53%   +0.01%     
==========================================
  Files          11       11              
  Lines         759      762       +3     
==========================================
+ Hits          725      728       +3     
  Misses         34       34              

Impacted Files	Coverage Δ
aiocogeo/constants.py	100.00% <ø> (ø)
aiocogeo/compression.py	89.06% <100.00%> (+0.53%)	⬆️

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