Improved control over output markup

Improvements:

* Allowing disabling the Menu Item Loop Markup (for example for singular Menu Item pages).
* Allow filtering the edit many menu items data before the posts are created. Useful for switching the 'post_content' for 'post_excerpt', for example. You can actually filter the data via 'wp_insert_post_data' filter, but this is easier, more convenient way.
* Allowing filtering menu item loop markup's opening and closing element method output as well as class setup method output.

Fixes issue [#1320](https://github.com/Automattic/jetpack/issues/1320)