Add WithAppErrorHandler

[izumin5210]

To close #5

[codecov-io]

Codecov Report

Merging #7 into master will not change coverage.
The diff coverage is 88.88%.

@@          Coverage Diff           @@
##           master      #7   +/-   ##
======================================
  Coverage    82.6%   82.6%           
======================================
  Files           3       3           
  Lines          46      46           
======================================
  Hits           38      38           
  Misses          7       7           
  Partials        1       1

Impacted Files	Coverage Δ
handlers.go	96.29% <88.88%> (ø)	⬆️

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