Implement an init command

[fastfrwrd]

We think it would be valuable to have a web-scripts init script which takes care of a few things.

• installs @spotify/web-scripts and husky as devDepdendencies if they are not currently a dependency or devDependency.
  • the package would need to install itself in case it was invoked via npx or was a global dep.
• adds npm scripts for build, test, lint, commit, and release if you don't already have them defined.
• adds the hooks for precommit and commitmsg in package.json (again, if neither is defined already)
• set main, module, and types in package.json to conform with web-scripts build's output.
• create tsconfig.json, .eslintrc.js, prettier.config.js, jest.config.js if they do not exist for your editor integrations, as described in the README.

We need to add an integration test for the init command.

[fastfrwrd]

create a separate npx-able like create-react-app or https://github.com/zeit/create-next-app