message: Add a polymer-3 linter collection.
  Currently it is just a clone of the polymer-2 recommended rules.
  Fixes
diff: @@ -17,6 +17,31 @@ import './rules';
  import {registry} from './registry';
  import {RuleCollection} from './rule';
  +registry.register(
  + new RuleCollection('polymer-3', `Rules for projects that use Polymer 3.x`, [
  + 'behaviors-spelling',
  + 'call-super-in-callbacks',
  + 'content-to-slot-declarations',
  + 'content-to-slot-usages',
  + 'content-selector-to-slotted',
  + 'create-element-extension',
  + 'custom-style-extension',
  + 'databind-with-unknown-property',
  + 'databinding-calls-must-be-functions',
  + 'deprecated-css-custom-property-syntax',
  + 'deprecated-shadow-dom-selectors',
  + 'dom-calls-to-native',
  + 'dom-module-invalid-attrs',
  + 'element-before-dom-module',
  + 'paper-toolbar-v1-to-v2',
  + 'root-selector-to-html',
  + 'set-unknown-attribute',
  + 'style-into-template',
  + 'unbalanced-polymer-delimiters',
  + 'undefined-elements',
  + 'validate-element-name',
  + ]));
  +
  registry.register(
  new RuleCollection('polymer-2', `Rules for projects that use Polymer 2.x`, [
  'behaviors-spelling',
  