New named index data structure

[standage]

This update adds a new NamedIndex class for retrieving features by ID.

• 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 #82 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #82   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           24        24           
  Lines         1362      1386   +24     
  Branches       304       310    +6     
=========================================
+ Hits          1362      1386   +24     

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

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