[RFC] Add twitter:card, twitter:site and twitter:creator tags

[michaelborn]

IMHO, the twitter support in this extension is a bit lacking. For a twitter URL to use the "nice" format, Twitter appears to require or at least suggest the following:

<meta name="twitter:card" content="summary" />
<meta name="twitter:site" content="@site_publisher_handle" />
<meta name="twitter:creator" content="@site_author_handle" />

See Twitter's documentation on social meta tags, as well as many others.

[michaelborn]

@bobdenotter Would you accept a PR for this? I believe these three meta tags would all have to be set in the config file, because they are site-generic and not specific to any page. That makes them simpler than, say, the og:title or <meta description...> because they don't need an editor UI control.