Add default 'create' command to create new app

[stevehanson]

Adds new default command to create a new app. Initializes with TypeScript, ESLint, Prettier, and creates basic app scaffold. It's still a little rough, not perfect, but we can iterate on that.

In the future, we will fill in this task to prompt about optional tooling, like state management, API clients, testing, and set those up as well.

Usage:

> thoughtbelt MyApp

To test this PR:

• Pull down this branch
• Run yarn link
• Create a directory somewhere else on your file system
• run yarn add thoughtbelt
• run yarn link thoughtbelt to use thoughtbelt from your local codebase
• run yarn thoughtbelt MyApp to run this command and see output!

thoughtbelt-create-app.mov