A few improvements / bug fixes

Hi there. Thanks for the plugin! I noticed a couple issues while using it, and needed to add one new feature for our use case.

Issues:
* Wording "Varnish purge flushed!" should be "Varnish cache purged!" (firejdl/varnish-http-purge@45051a1)
* Whole site was being purged when a page/post is saved, because the 'save_post' action is fired for revisions as well (firejdl/varnish-http-purge@5446921)
* Existing query args were being ignored in the "Purge Varnish" button URL, so you were taken to a different page after purging (firejdl/varnish-http-purge@b6233b3)

Feature:
* Run action hook 'after_purge_url' after purging a url (firejdl/varnish-http-purge@81ac6b9)