filter_pcre: Added a plugin to filter value lists based on PCRE.

filter_pcre: Added a plugin to filter value lists based on PCRE.

The user may specify a set of Perl-compatible regular expressions to match any
component of the (host, plugin, plugin instance, type, type instance) tuple.
Any of the filter flags may be used to handle a successful match.

Sample plugin configuration:

  <Plugin filter_pcre>
    <RegEx>
      Host "^mail\d+$"
      Plugin "^tcpconns$"
      TypeInstance "^SYN_"

      Action NoWrite
  </Plugin>