add codecov integration

[mthenw]

Coveralls is super slow and configuring is a bummer.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@7be10b0). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #391   +/-   ##
=========================================
  Coverage          ?   57.09%           
=========================================
  Files             ?       24           
  Lines             ?     1389           
  Branches          ?        0           
=========================================
  Hits              ?      793           
  Misses            ?      551           
  Partials          ?       45

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