Add support for Unix Domain Sockets

[shaharke]

Datadog documentation recommends using UDS (also known as IPC in Node.JS) when working with the Datadog containerized Agent (see https://docs.datadoghq.com/developers/dogstatsd/unix_socket). There's also a guide for implementing UDS in client libraries: https://github.com/DataDog/datadog-agent/wiki/Unix-Domain-Sockets-support.

In its current implementation this library support only UDP/Datagram sockets.