Allow markdown for generating XHTML files

[smhmd]

Right now, my process to generate files is: get an epub, covert it to a txt file, run it through syncabook, make it an epub again. By doing this, I lose a lot of information from the original epub like italicized and bold text, links, images, etc. I imagine we can instead of converting from epub to txt, convert to markdown to retain some of that info. Ofc, parsing txt for sentences isn't the same as parsing markdown, so this is probably not a straightforward feature, I imagine.

Would love to hear your thoughts on this.