Make better use of title and description

To make the most out of meta tags as well as opengraph and twitter_cards templates:

• Use .Site.Params.description as the site description;
• For title:
  • On the home page: Use only the page title, so title in content/_index.md should be set differently (only "should be", not "must be");
  • On other pages: <page title> - <site title>, so title in config.yaml should be set to only the application name.

@carlschwan