Sharing: add new filter to turn off sharing counts

Some site owners don't want sharing counts, because of the performance hit caused by the 3 external requests to Twitter, LinkedIn, and Facebook.

The existing `sharing_js` filter disables all the javascript and removes the sharing counts, but also breaks the email sharing button.

This new filter, `jetpack_sharing_counts`, disables only the sharing counts, without breaking the email button.
If you want to check if `sharing_js_options.counts` is `true` I would write this line as  `if ( sharing_js_options &&  sharing_js_options.counts )`