Add support for "global" tags that are added to all tags.

This change adds another (optional) argument to `enable()` that allows the user to provide a list of tags that will be included with all metrics sent to Datadog. I foresee using this with things like application versions or names!

I also included documentation for the existing tag support. I believe I read the source right so that I could explain it. Thanks and let me know if I can help to get this merged by making any changes.  Thank for making this!