Merge branch 'keep-cached-images-fresh' into 'master'

Keep local cached images fresh if file exists.

Updates the cache_images() function to `touch` images if the file exists already. Some sites (e.g. news) re-use stock images and this change updates the local file's modified time every time the image is referenced in new articles. This should prevent the local file from being expired/deleted only to be re-cached later on.

See merge request !9