New pep2nuc command

[standage]

This update adds a new tag pep2nuc command for transforming feature coordinates from peptide space to nucleotide space.

• Full test coverage? Any additional tests that could easily be added?
• API documentation up-to-date? Docstring examples?
• Is the CHANGELOG up-to-date?

[codecov]

Codecov Report

Merging #83 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #83   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        25    +1     
  Lines         1386      1423   +37     
  Branches       310       313    +3     
=========================================
+ Hits          1386      1423   +37     

Impacted Files	Coverage Δ
tag/cli/__init__.py	100.00% <100.00%> (ø)
tag/cli/pep2nuc.py	100.00% <100.00%> (ø)
tag/index.py	100.00% <100.00%> (ø)

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