Improve semantic markup of site and posts

[happybeing]

v0.11 introduced some basic RDFa markup of Plume posts in the header metadata for the page.

This can be enhanced by:

• adding support for twitter:cards (the code is there but we need to test getting this from the WebID profile document)
• adding an image from the post itself (currently it defaults to the Plume logo)
• extending the schema.org markup to post content (see Ruben Verborgh's blog posts as an example). I think this would be added in app.js/postToHTML().
• provide some basic metadata for other pages (so far only the page for viewing a single blog post has semantic markup, not the home page for example)

There are probably other related improvements as the initial implementation was my first encounter with adding semantic markup.