allow selecting entries /not/ matching an attribute

there are various kinds of "@incollections" papers, e.g. workshop papers, which I don't want to put in the same list as other papers on my website. This commit allows to use an attribute like a tag list, where you can select or exclude entries.

On the choice of operator (`%=`): I'm not too fond of it, I'd prefer to use perl notation (`=~` and `!~`), but that would break compatibility. Feel free to suggest an alternative.