Support configuring the client from environment variables

[ahmed-mez]

All of the official Datadog dogstatsd clients can now be configured via standard environment variables. As the maintainer of the haskell dogstatsd library, could you port the same functionality to this client? We would prefer that configuration be consistent across client libraries.

Sample changes from the Go implementation can be found here: DataDog/datadog-go#78

Let me know if you have any questions about the code or the feature more generally.

Thanks,

Ahmed