Properly sideload child images and account for some common use cases when converting images

[srtfisher]

Fixes #26

• Ensures that child images are properly sideloaded.
• Account for converting a <figure> and other common uses to an image block.
• Add collection of attachments that are created during the conversion process.
  Two methods now exist on the converter to help:
  • get_created_attachment_ids() - Returns the attachment IDs that were created.
  • assign_parent_to_attachments() - Assigns a parent post to all attachments.

[attackant]

🐙