Enhance filter option handling for HTML image embedding support

[kracas]

The default behaviour of embedding images as base64 strings in output HTML files was changed for newer libreoffice versions. This was then made optional and you could specify this in the soffice commands like so:

soffice –headless --convert-to html:HTML:EmbedImages test_file.docx

However using --filter-options EmbedImages=true does not work, because it sets FilterData property, whereas for EmbedImages option FilterOptions property needs to be set to EmbedImages.

This PR makes it possible to use EmbedImages filter option this way:
unoconvert --convert-to html --filter HTML --filter-options EmbedImages=true test_file.docx test_file.html
or just
unoconvert --convert-to html --filter HTML --filter-options EmbedImages= test_file.docx test_file.html

Fixes #24 #75 #110

[regebro]

OK, I understand what is happening here now, I implemented this in a more generic fashion. Version 3.1 is out now.