Make it possible to set publicPath on build

[diegohaz]

This PR adds the possibility to set the webpack publicPath config and react-router basename on build by just running PUBLIC_PATH=foo npm run build.

This is necessary to run the project on URLs like https://diegohaz.github.io/foo

[codecov-io]

Codecov Report

Merging #97 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #97   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          39     39           
  Lines         257    257           
  Branches       51     51           
=====================================
  Hits          257    257

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