Add Twitter/Facebook social cards

[cvan]

After the graphics have been created, we need to add these HTML elements.

I put the URL as https://lobby.webvr.rocks/, but I'll leave that up for discussion.

<meta property="og:title" content="WebVR Rocks">
<meta property="og:site_name" content="WebVR Rocks">
<meta property="og:type" content="website">
<meta property="og:url" content="https://lobby.webvr.rocks/">
<meta property="og:description" content="Your guide to VR on the Web.">
<meta property="og:image" content="https://lobby.webvr.rocks/card.png">
<meta name="twitter:card" content="summary">
<meta name="twitter:site" content="@WebVRRocks">
<meta name="twitter:creator" content="@WebVRRocks">
<meta name="twitter:title" content="WebVR Rocks">
<meta name="twitter:description" content="Your guide to VR on the Web.">
<meta name="twitter:image" content="https://lobby.webvr.rocks/card.png">