JJG tweaks

Several fixes and enhancements
- removed class variables, replaced by `app.set`
- fixed lost `convert_latex` (result is returned, does not modify in-place)
- allow more flexible search, in particular all of the entries in the file
- made the unformatted entry accessible, so it can be interrogated (README has example of use)
- public menthods are `citation_...` or `citations_..,` to match the extension-name