message: update changelog for v0.18.0
diff: # Changelog
  -<!--## Unreleased-->
  +## v0.18.0 [04-13-2017]
  -- `build`: Added `--add-push-manifest`/`addPushManifest` option for generating a [`push-manifest.json`](https://github.com/GoogleChrome/http2-push-manifest) file for your project.
  -- `build`: Fixed a bug where `--insert-prefetch-links` would generate 404ing imports.
  +- `build`: Add `--add-push-manifest`/`addPushManifest` option for generating a [`push-manifest.json`](https://github.com/GoogleChrome/http2-push-manifest) file for your project.
  +- `build`: Fix a bug where `--insert-prefetch-links` would generate 404ing imports.
  - `build`: Update automatic `webcomponentsjs` polyfilling to move it and all affected elements following it into the body so that the `custom-elements-es5-adapter.js` can work properly in IE11. (See [#627](https://github.com/Polymer/polymer-cli/issues/627))
  - `init`: Init template elements now properly inherit from the given element/app name.
  -- `init`: For `polymer-2-element` template, remove iron-component-page from the served `index.html` file until it can support Polymer 2.0 class-based elements.
  -- `init`: Update polymer 2.0 application & element tests to improve and fix broken tests
  +- `init`: Fix `polymer-2-element` template serving by removing iron-component-page until it can support Polymer 2.0 class-based elements.
  +- `init`: Update polymer 2.0 application & element tests to improve and fix broken tests.
  - `init`: Update polymer 1.x application & element template WCT dependency to `^6.0.0-prerelease.5`.
  -- `init`: Update polymer application & element READMEs
  -- `serve`: Update to polyserve@v0.17.0 to support autocompilation when serving to Chromium, Edge browsers
  +- `init`: Update polymer application & element READMEs.
  +- `serve`: Update to polyserve@v0.17.0 to support autocompilation when serving to Chromium, Edge browsers.
  - [Breaking] Remove Node v4 support: Node v4 is no longer in Active LTS, so as per the [Polymer Tools Node.js Support Policy](https://www.polymer-project.org/2.0/docs/tools/node-support) the Polymer CLI will not support Node v4. Please update to Node v6 or later to continue using the latest verisons of Polymer tooling.
  ## v0.18.0-pre.15 [03-22-2017]
  