An extension starter project

[bajtos]

Create loopback-next-hello-extension (or similar name), a github project that people can git clone and start developing a new extension.

Ideally, the sample project should have placeholders/templates for all kinds of things that an extension can contribute:

• providers
• controllers
• repositories
• a mixin adding application methods and configuration

The idea is that as a new extension developer, I can fill in my implementation in the placeholders, remove placeholders for things I don't need, and have the extension ready in no time.

IMO, the project should be using TypeScript, have all necessary npm scripts already configured (build, test, etc.) and ideally include a reasonable test suite too.

See #218