Job definition filter implementation

This is an implementation of what @mrdavidlaing proposed in his [comment](https://github.com/logsearch/logsearch-boshrelease/issues/9#issuecomment-41587591).

Users are able to update the `logstash_parser.filters` config with URLs of their tar/tar.gz/zip files. Files are only downloaded once and are extracted in order of appearance within the array value. This means a later file could overwrite an earlier, upstream version if using the exact same name (which I consider a feature). Once extracted, they're loaded into the logstash configuration file still in order of priority based on the name.