support for external hyperlinks

A few things going on in this commit; everything related to adding support for carrying over hyperlinks from a word document to PB. 

- cleaned up some formatting
- renamed some functions `getIDs()` and `getRelationshipPart()` to reflect a broader range of functionality that they now support.
- cleaned up some logic in those functions.
- added two functions `addHyperlinks()` `addFootnotes()` 
- added support for hyperlinks as outlined in issue no. 5  https://github.com/bdolor/pressbooks/issues/5