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 C# 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 C# implementation can be found here: https://github.com/DataDog/dogstatsd-csharp-client/pull/78

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

Thanks,

Ahmed