#725 Another modifier for bibtex key generator

[Sergiy Bubin]

I belive this one should be really easy to implement. In bibtex automatic key generator it would be nice to have a modifier similar to "abbr", which in addition to abbreviating could also discount words "the" and "of".
For example, when I apply [journal:abbr:lower] to "The Journal of Chemical Physics", it generates "tjocp". A more meaningful abbreviation would be "jcp", not "tjocp".