Feature/update sudoku exercise (#23)
* Updated sudoku exercise
- Moved from custom webpack configuration to use react-scripts to make things simpler.
- Upgraded dependencies to latest ones.
- Added linting and integration with codacy/codecov.
- Added continuous integration examples of how to run tests on push/pull requests and how to "deploy" in github pages upon release.
- Updated dockerfile to use an official node image so we don't have to install everything from scratch.
- Updated sass examples to make them more interesting.
