fix(index): open store, prior to read to not overwrite store.

[bplaster]

Fixes #17

[codecov-io]

Codecov Report

Merging #18 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   97.75%   97.77%   +0.02%     
==========================================
  Files           1        1              
  Lines          89       90       +1     
==========================================
+ Hits           87       88       +1     
  Misses          2        2

Impacted Files	Coverage Δ
index.js	97.77% <100%> (+0.02%)	⬆️

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