Add WP-CLI command for rewriting old posts

[joemcgill]

Another approach to providing responsive images to posts written prior to installing this plugin, is to provide a command through WP-CLI which would update all of the posts in the database with the new image markup. Ideally, we would do a search of the DB for all instances of images with a URL path that matches the site's uploads directory (or by looking for a wp-image- class) and update each of those posts so that they can be rolled back, via post revisions, if anything breaks.

[joemcgill]

Related to issue #83

[danjaworsky]

+1

[ryanwiemer]

+1 I really hope this can be implemented in the near future.

[jasperf]

+1 Would really be great. Just added the plugin and the blog is atm loading several older posts that do not have the images that are responsive.

[joemcgill]

Since we've switched to a display filter, this functionality is no longer needed. Related #148.