Handle internal links in XHTML export

Rewrites internal links in the XHTML export. So links in the content,
references and lists work.
fixes #6
Bugfix: Epub/Hpub links to draft chapter now work

Links to posts that are draft are not transformed to the slug form. The
links look like ?p=…. Up till now this was not recognized as an intern
link and there for was not altered to work in the Epub and Hpub.
Merge branch 'dev' of https://github.com/lukaiser/pressbooks-lists into dev