Add AttachmentToHTML library

Extracts the attachment processing from FoiAttachment#body_to_html

AttachmentToHTML contains adapters which convert
  - text/plain
  - application/pdf
  - application/rtf

Results are returned as an AttachmentHTML::HTML instance which contains
the raw HTML and other metadata about the conversion.
Use AttachmentToHTML to generate FoiAttachment#body_as_html