[HELP WANTED] Add types for TS/Flow

Add type annotations for TypeScript & Flow.

As of 3.7.0-beta.1, the generator runs template files through EJS, then Babel, and then a beutifier before writing them to your scaffold directory. This means that now we can include things like type definitions, and have an option to include them or not. You can see the first type annotation I added here: auth.service.js#L23.

It would be a fairly large undertaking to manually add type annotations to every file in the whole project, so I'm looking to the community for help. Here's what to do if you'd like to help:

• branch off of the canary branch
• templates are located in the templates folder.
• it would be easiest if type definitions submitted work with both TypeScript and Flow
• you can include TS/Flow specific code by using EJS (use filters.ts and filters.flow)

<%_ if(filters.ts) { _%>
interface thing {
  func(value: any): string;
}<% } %>

Comment here if you'd like to help out.