Add shareable configuration

[remcohaszing]

As pointed out by @AdriVanHoudt in #258 it might be a nice idea to have a shareable configuration.

New rules added to these defaults, would mean breakage though. Not adding these rules would mean an outdated config.

Upgrading the major version for each new rule seems overkill though.

I suggest to add multiple inheritable configs:

extends:
  - plugin:eslint-plugin-angular/johnpapa
  - plugin:eslint-plugin-angular/bestpractices
  - plugin:eslint-plugin-angular/recommended

Where:

• johnpapa enables rules related to John Papa's style guide and new rules to enforce John Papa's style guide are not considered breaking changes.
• bestpractices enables rules enforcing the AngularJS best practices.
• recommended enables some recommended rules and will not change until a new major release is made. E.g. it enables di-unused.

I'm not sure how the prefixes required by some rules should be handled.

[AdriVanHoudt]

It would only be breaking if you add rules that you would want to enforce in the default but yeah. The multiple config idea sounds good

[dustinspecker]

If the maintainers of this plugin are interested, I would be more than glad to add contributors to eslint-config-angular on both GitHub and npm. I haven't been using Angular much recently, so I've definitely been doing a poor job of keeping the config up to date with this plugin.