External navigation via hyperlink

[jrbowden]

Related to #9 and #45, hyperlinks in documents could point to other documents or the same file type.
For example assume a collection of documents is delivered with a master index file. The master index could then contain a brief summary of each book and a link to open the book.

Thus we now have (at least) three possibilities for a hyperlink:

1. points somewhere in the same file in the same book - e.g. a footnote contained in the same chapter.
2. points somewhere in a different file in the same book - e.g. link from a contents to a chapter, or a "see also" which points to another chapter.
3. points to a different book - e.g. a master index opening a different book.

If needed this could be extended to allow:
4. points to a website or other external drive containing a book - e.g. book is online.
5. points to another file type entirely - e.g. opens a graphics file or other resource that the reading system is capable of rendering.

Proposal: a hyperlink could point to another (local) file.