BREAKING CHANGE: drop support for versions of Node which are EOL'd as of 2019-04-30.
* chore: upgrade mocha to fix some devDeps issues with npm audit
* chore: upgrade commander and electron deps
* chore: upgrade electron-mocha, which requires Node >= 8
* docs: update code examples for Node 8
* refactor: replace pify with util.promisify
* refactor: use async/await everywhere
* chore: remove Node 6 from the Node versions tested in CI
* refactor: use async readFile method
* refactor: simplify pattern construction
* docs: reformat createPackageFromFiles doc comment
* refactor: DRY up determineFileType
* chore: remove mz devDependency