add cleanup option to remove old indexes that are no longer used

[nicpottier]

No description provided.

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by 1.48%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   78.73%   77.24%   -1.49%     
==========================================
  Files           1        1              
  Lines         174      189      +15     
==========================================
+ Hits          137      146       +9     
- Misses         21       24       +3     
- Partials       16       19       +3

Impacted Files	Coverage Δ
indexer.go	77.24% <60%> (-1.49%)	⬇️

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