add function to extract media to library

According to this manual, media files appear either inside figure or span wrapper nodes and they all have the attribute typeof = "mw:File/*" . Additionally, the specific information related to specific media types can be found inside specialized nodes, i.e:

• Images -> <img>
• Video -> <video>
• Audio -> <audio>

However, upto HTML 2.4.0 there were separate mw attributes for each type of files, i.e: mw: image, mw:audio, mw:video . See this issue for reference.