Add full support for image, video and audio tags.

[ItayZiv]

Closes #53. Adds full support for image, video, and audio og tags:

• [x] Relative path support for images
• [ ] Relative path support for video and audio
• [ ] Automatic mime type detection and validation
• [ ] Width/Height tag support for images
• [ ] Width/Height tag support for video

Currently, whether an image is configured in conf.py or in field lists, if it starts with / it is treated as relative to the root, if it doesn't it's treated relative to the current document. I think this functionality is fine and allows you to set up different images for each file without field lists if you use the same naming scheme (i.e. `images/docname/opengraph.png). but open to comments.

[Daltz333]

Is this PR stale?