Update options.js

I changed the original options.js file line 61 (now line 65) to include 'extra' as a sort option on the menu.

I also had to manually update the key @Preferences Menu, Advanced, Miscellaneous, Open about:config
Key to update: extensions.zotero-report-customizer.sort
To the beginning of the key array, I added after the "[": {"name":"extra","order":"a"},
So now my whole key is:
[{"name":"extra","order":"a"},{"name":"dateModified"},{"name":"firstCreator"},{"name":"title"},{"name":"pages"},{"name":"date"},{"name":"accessed"},{"name":"dateAdded"},{"name":"publicationTitle"},{"name":"publisher"},{"name":"itemType"},{"name":"series"},{"name":"type"},{"name":"medium"},{"name":"callNumber"},{"name":"archiveLocation"},{"name":"DOI"},{"name":"ISBN"},{"name":"ISSN"},{"name":"edition"},{"name":"url"},{"name":"rights"}]

Maybe there's a way to automate updating the key, but this work-around got me on my way.
Merge pull request #6 from birch96/master

Update options.js