Filters URLs based on a file of URL prefixes. The file is named by (1)
property "urlfilter.prefix.file" in ./conf/nutch-default.xml, and (2)
attribute "file" in plugin.xml of this plugin Attribute "file" has higher
precedence if defined.
The format of this file is one URL prefix per line.
Fields inherited from interface org.apache.nutch.net.URLFilter