ng add @angular/seo

_From @IgorMinar on April 18, 2018 10:7_

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Area
```
- [ ] devkit
- [x] schematics
```

### Versions
v6

### Desired functionality
I'd like to be able to invoke `ng add @angular/seo` and have the CLI make my application search engine friendly. Initially I'd like to see the following feature provided by the schematics:

- generate robots.txt with no rules but a simple instructions/link on how to setup blacklist rules
- generate sitemap.xml with similar documentation/links as in robots.txt
- add code that would feature detect and conditionally load polyfills needed by the crawlers
- setup error logging via global error handler with an option to redirect the output to a remote server
- provide facilities to easily add and remove the `noindex` metadata to emulate the HTTP 404 and 500 status codes

### Mention any other details that might be useful

_Copied from original issue: angular/devkit#729_

Commencez par examiner la zone des schematics de Angular CLI et le flux de `ng add` décrit dans l’issue. Décomposez la demande en fonctionnalités SEO listées—robots.txt, sitemap.xml, polyfills pour les crawlers, journalisation des erreurs et gestion de noindex—et clarifiez le périmètre et les critères d’acceptation avant de modifier quoi que ce soit ; aucun fichier ni test n’est nommé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.