Improved control over Nova Menu output markup

**Improvements:**

* Allowing disabling the Menu Item Loop Markup (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, I think.)_
* 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)**