Support showing the eprint bibtex entry in the cite panel

This adds support for an "eprint" keyword for settings such as cite_panel_format, so that the bibtex eprint entry can be displayed in the cite panel etc. The bibtex entry is wrapped in angular brackets [...] if it is not empty, just as they are often displayed in publications (doing this by hand in cite_panel_format would lead to many [] for all those bibtex entries that don't have an eprint associated with them).