Add ability to convert relative links to absolute
Add footnote prefix option. Needs testing
Use parameters object for extra options. Enhance footnote support.

Option to add return links.
Option to make footnote prefixes unique, for rendering multiple
documents per page.
Move footnote prefix to a better place
Add tests for new footnote functionality
Add tests for absolute prefix
Fix spelling error
Merge pull request #86 from dimfeld/master

Add absolute link transformation and footnote enhancements