Facilitate link rewriting

I'm not sure if this should be done from this library or if it should be done in the application code that uses this library, but at some point (in Yakread's case at least), anchor links and image sources need to be rewritten so that they work in a web app. Maybe we'd want to add some options to `parse-epub-dir` so it can help with that in one way or another. (I haven't thought about it much at all yet.)

Start by examining the library entry point mentioned in the issue, `parse-epub-dir`, and how Yakread currently consumes anchor links and image sources. Determine whether rewriting belongs in this library or application code, then define the needed options and expected behavior before implementing and testing the chosen approach.

Written by the indexing model from the issue text.