Add ability to add custom tags to all metrics.

[Stovoy]

This is useful when you need to differentiate between multiple different types of services running through one dogstatsd agent.

[remeh]

Sorry for the very late reply and thanks for your contribution @Stovoy
The custom tags feature has been merged with #17 in master. The configuration field for tags is called dogstatsd-extra-tags, I hope that this field name change won't be a problem to you. Once again thanks for your contribution and sorry for the delay.