Update badges to use shields

[blakeembrey]

Consistent badges with other repos like path-to-regexp.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (3817c1a) to head (dfd97e7).

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          120       120           
  Branches        39        39           
=========================================
  Hits           120       120           

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