add OpenGraph tags to base template

Provides the following blocks to override the content of each meta tag and provides sensible defaults when it makes sense.

* `og_image`
* `og_title`
* `og_url`
* `og_site_name`
* `og_type`
add twitter card meta tag
Merge pull request #6 from marquee/feature/og-tags

add OpenGraph tags to base template