Add support for media targets

[fluffy-critter]

<img src>, <video src>, etc. should also send a webmention. For media targets it should only be necessary to HEAD, rather than GET, the resource, and <script src> should probably be excluded. There is also no reason to look at rel on this.

[fluffy-critter]

Low priority because I have no evidence that anyone even bothers to receive/display webmentions on images at this time.