Add per-collector ability to whitelist/blacklist metrics

- Configured with keys metrics_whitelist and metrics_blacklist
  to the collector's section in the config file
- Keys aren't regexes, straight strings that are comma separated
- Providing both a whitelist and a blacklist will result in an
  exception being thrown
Use a regex for metrics_blacklist and whitelist
Merge pull request #672 from yuvipanda/whitelist

Add per-collector ability to whitelist/blacklist metrics