Add hOCR serializer

The hOCR format is a specific definition of a HTML format. The existing HTML serializer might be a good starting point.

https://en.wikipedia.org/wiki/HOCR

The hOCR format is most commonly used in order to make searchable PDF files or as an extracted metadata of the PDF file. In order to create searchable PDF files we can use a scanned document image and a .hocr file of the particular image. With the help of open source tools (hocr-tools, hocr2pdf) these files can be merged to a PDF file.