Add WPEngine detection

I would find it useful to know whether a WordPress website I am looking at is hosted on WPEngine.

I expect this can be pretty reliably detected by checking whether a file such as `/wp-content/mu-plugins/wpengine-common/changelog.txt` 200's, since this is a plugin WPEngine will install on all builds it hosts, it will require an AJAX request however, so maybe only scope this check if WordPress has already been detected.

Thoughts?