Customize Posts free WordPress plugin

This is a feature plugin intended to implement #34923: Introduce basic content authorship in the Customizer. The goal for this plugin is to be able to expose the editing of posts and pages in the Customizer, allowing you to edit post data and postmeta for any number of posts, and preview the changes before saving them for others to see. This plugin was birthed out of the Widget Customizer feature-as-plugin project which was merged into WordPress Core: as widgets (in 3.9) and nav menus (4.3) can now be managed in the Customizer, so too should posts and pages be editable in the Customizer as well. Did you know that changing the featured image actually makes the change live even before you save the post? This is very surprising/unexpected behavior. The only way to truly preview a change to a featured image is to use something like Customize Posts. Likewise, did you know that changing a page template cannot be previewed from the post editor? When you change the selected page template, the change will not show up when you preview the page (see #11049). However, in Customize Posts you can preview changes to the page template just by changing the dropdown selection, and then you can see what your page would look like with the new template after the preview refreshes. Most other changes to metaboxes containing data that gets saved to custom fields (postmeta) also get written when clicking the Preview button. The Customize Posts plugin provides a framework to edit postmeta in the Customizer with a live preview of the changes. (Fixing this underlying issue of incorrectly persisting postmeta when doing a preview is captured in #20299.) As much as possible, the previewing of changes in Customize Posts utilizes the selective refresh capabilities introduced in WordPress 4.5. Not only does this mean it is