Polynomial Interpolation Web GUI with GeoGebra

Live version at https://comfreek.github.io/polynomial-interpolation-web-gui/.

Dev Notes

• We use GeoGebra Apps Embedding and its corresponding GeoGebra Apps API.
• npm run lint: Lint using ESLint
• npm test: Run integration tests using Cypress. (As set by package.json, Chrome is hardcoded as the browser due to issue #2.)
• node check-geogebra-apps-version.js: Check whether we use the latest GeoGebra apps version and return a non-zero exit code if not. The script is used to fail the Travis build, which is set up to run every 2 weeks -- apart from regular commits. On failure, the repository's owner (that's me!) gets an e-mail. Thus, we more or less fake Greenkeeper behavior for GeoGebra Apps, for which no officially maintained NPM package exists.