tagging dev-only code (@remove-on-publish-begin/end) and pre-eject-only code (@remove-on-eject-begin/end) #257 (#257)

This makes generators work with Jest.

Fixes #255.
Replaces #262.

Disabled because of issues:
 * https://github.com/facebookincubator/create-react-app/issues/525
 * https://phabricator.babeljs.io/search/query/pCNlnC2xzwzx/

TODO: Enable again when these issues are resolved.

* Started using babel-preset-latest

* Updated babel dependencies

* Remove redundant babel-plugin-syntax-async-functions

* Disabled redundant regenerator transform

* Transform async functions with regenerator

Remove `transform-async-to-generator`, which to my understanding is meant
to be used in environments that support generators natively.

Because we're compiling generators to ES5 anyway, we can simply use
`regenerator` to transform async functions too, which results in
slightly simpler output and only uses the regenerator runtime instead
regenerator runtime + _asyncToGenerator Babel helper.

* Add babel-plugin-transform-regenerator

* Use dependencies with an exact version

* Make Babel config more verbose and simpler

* Support async/await

* Enable generators

* Opt out of using helpers

Fixes #31

* Use a custom eslint rule list

* Sort rules

* Relax some rules