**Summary**
Allow registration of a custom sub-partitioner that extracts images from
a DOCX paragraph.
**Additional Context**
- A custom image sub-partitioner must implement the
`PicturePartitionerT` interface defined in this PR. Basically have an
`.iter_elements()` classmethod that takes the paragraph and generates
zero or more `Image` elements from it.
- The custom image sub-partitioner must be registered by passing the
class to `register_picture_partitioner()`.
- The default image sub-partitioner is `_NullPicturePartitioner` that
does nothing.
- The registered picture partitioner is called once for each paragraph.
