Add meta tag support

[simanerush]

• add meta tag support for individual pages

  ...
  extra:
  meta: 
  - property: og:title 
    content: the title of the page 
  - property: og:description
    content: the description of the page 

  The YAML code above will generate <meta property="og:title" content="the title of the page"/> and <meta property="og:description" content="the description of the page"/>.

• default the og:title and og:description to the pages' title and description.

[not-matthias]

Thank you!