blacklist IDs, mention xpath as param

fixing #1 and #2
e.g denormalize indices but exclude Schnitzler, Bahr and Wien
$ denormalize-indices -f "./data/*/*.xml" -i "./data/indices/*.xml" -m ".//*[@key]/@key" -x ".//tei:title[@level='a']/text()" -b pmb2121 -b pmb10815 -b pmb50