Allow filtering the Horizon meta tags before output.

This change allows a developer to customize or completely disable the output of the Horizon meta tags.

Following the pattern established in the [Jetpack's Open Graph meta tag
support](http://plugins.svn.wordpress.org/jetpack/tags/2.4.2/functions.opengraph.php), this change introduces three new filters with the aim to allow
developers to customize the output of horizon meta tags.

**sailthru_horizon_meta_tags_enable** - return boolean false to prevent the
horizon meta tags from outputting. Useful when you wish to rely on
the fall back meta tags or manually output your own.

**sailthru_horizon_meta_tags** - is supplied an array of the
name/content attributes of the meta tags. This is helpful if, for example, you need to update the value of sailthru.author when using a plugin like co-authors-plus, or for swapping out sailthru.tags for categories or a terms from a custom taxonomy.

**sailthru_horizon_meta_tags_output** - this filter acts on each meta tag,
allowing you to edit each html tag.