docx: include embedded images as Image elements

[scanny]

An author can embed one or more images in a Word document.

Extract those during partitioning and include them in the element stream as an Image element if the partition strategy is "hi_res".

[MthwRobinson]

@scanny - Have you done this for Word docs already or is that still in progress?

[scanny]

@MthwRobinson still WIP.

[scanny]

Added in the API a couple months back.