Use eslint-config-koa

[nickmccurdy]

https://github.com/koajs/eslint-config-koa

[codecov]

Codecov Report

Merging #1105 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1105   +/-   ##
=======================================
  Coverage   99.73%   99.73%           
=======================================
  Files           5        5           
  Lines         371      371           
=======================================
  Hits          370      370           
  Misses          1        1

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[nickmccurdy]

It works well with the current versions of eslint plugins, but the latest versions of eslint-plugin-standard and its peerDependencies are also supported by eslint-config-koa. Do we want to merge now and shelve the package updates for later or would you like me to do it in this pull request?