(ALSO NOT FOR MERGING) use Yarn workspaces and Babel 7

[kohlmannj]

Based on #3. I wanted to riff on @rosskevin and @RyanCavanaugh's work by configuring this monorepo to not only use Yarn workspaces, but also use Babel 7 for TypeScript transpilation. This setup closely resembles an internal monorepo I've been working with.

Notable Changes

• Update README with usage and setup details
• Add multiple @babel/* devDependencies to the root package.json
• Add @babel/cli and concurrently devDependencies to each package
• Upgrade typescript devDependency refspecs to ^3.0.0-rc
• tsconfig.settings.json: add "emitDeclarationOnly": true
• Add scripts to root package.json:
  • bootstrap (run this instead of yarn install && lerna bootstrap)
  • build (uses concurrently)
  • build:js
  • build:dts
  • clean (uses concurrently)
  • clean:lerna
  • clean:lib
  • dev (uses concurrently)
• Add scripts to each package:
  • build (uses concurrently)
  • build:js
  • build:dts
  • dev (uses concurrently)