Add Feature Metadata to writing gct file

[gwaybio]

gct files support metadata for features as well as samples. This PR adds this functionality

[codecov-io]

Codecov Report

Merging #68 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   97.28%   97.32%   +0.04%     
==========================================
  Files          40       40              
  Lines        1731     1760      +29     
==========================================
+ Hits         1684     1713      +29     
  Misses         47       47

Impacted Files	Coverage Δ
pycytominer/tests/test_write_gct.py	100% <100%> (ø)	⬆️
pycytominer/write_gct.py	100% <100%> (ø)	⬆️

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