Better filtering of horizon meta tags

Following the pattern established in the Jetpack's Open Graph meta tag
support, 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 and the
horizon meta tags will not be output.  Useful when you wish to rely on
the fall back meta tags or manually output your own seperate from the
plugin.

sailthru_horizon_meta_tags - is supplied a key value pair of the
name/content attributes of the meta tags. This makes editing the
content of the meta tags very easy.

sailthru_horizon_meta_tags_output - this filter acts on each meta tag,
allowing you to edit each html tag.