creates a class feature_attribute and configures it to handle QTLDB.gff files.

[azneto]

This pull request addresses issue #...

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the terms of the GNU General Public License version 3 as published by
the Free Software Foundation

I have read the CONTRIBUTING.rst file and understand that
TravisCI will be used to confirm the tests and flake8 style
checks pass with these changes.

I am happy be thanked by name in the CONTRIB.rst files,
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[codecov-commenter]

Codecov Report

Merging #307 (c996fe3) into master (0c23682) will increase coverage by 0.06%.
The diff coverage is 72.54%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
+ Coverage   69.25%   69.32%   +0.06%     
==========================================
  Files          29       30       +1     
  Lines        3968     4019      +51     
  Branches      226      227       +1     
==========================================
+ Hits         2748     2786      +38     
- Misses       1161     1175      +14     
+ Partials       59       58       -1     

Impacted Files	Coverage Δ
machado/search_indexes.py	0.00% <0.00%> (ø)
machado/decorators.py	35.53% <23.52%> (-0.58%)	⬇️
machado/loaders/feature.py	65.46% <83.33%> (-1.54%)	⬇️
machado/loaders/featureattributes.py	84.09% <84.09%> (ø)
machado/loaders/publication.py	86.11% <100.00%> (ø)
machado/views/common.py	68.75% <0.00%> (ø)

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