Customize Snapshots
by Weston Ruter on WordPress.org
Provide a UI for managing Customizer changesets; save changesets as named drafts, schedule for publishing; inspect in admin and preview on frontend.

The “Save & Publish” button becomes a combo-button that allows you to select the status for the changeset when saving. In addition to publishing, a changeset can be saved as a permanent draft (as opposed to just an auto-draft), pending review, scheduled for future publishing. A revision is made each time you press the button.
Customize Snapshots is the feature plugin which prototyped Customizer changesets; this feature was merged as part of WordPress 4.7. The term “snapshots” was chosen because the Customizer feature revolved around saving the state (taking a snapshot) of the Customizer at a given time so that the changes could be saved as a draft and scheduled for future publishing.
While the plugin’s technical infrastructure for changesets was merged in WordPress 4.7, the user interface still remains largely in the Customize Snapshots plugin, in which we will continue to iterate and prototype features to merge into core.
For a rundown of all the features, see the screenshots below as well as the 0.6 release video:
This plugin works particularly well with Customizer Browser History, which ensures that URL in the browser corresponds to the current panel/section/control that is expanded, as well as the current URL and device being previewed.
Requires PHP 5.3+. Development of this plugin is done on GitHub. Pull requests welcome. Please see issues reported there before going to the plugin forum.