filter_pcre plugin: Added support to rewrite value lists.

filter_pcre plugin: Added support to rewrite value lists.

For this purpose "Substitute*" options have been introduced for each regex
option. Upon a successful match, the matching substring will be replaced by
the specified replacement text.

Configuration example:

  <Plugin filter_pcre>
    <RegEx>
      Plugin "^sensors$"
      PluginInstance "^Some Weird Sensor Chip Name Prefix"

      SubstitutePluginInstance "foo"
    </RegEx>
  </Plugin>