Adding tags to the email

[damianbl]

1. What version of the module is the issue happening on? Does the issue happen on latest version?
   Latest version

2. What platform and Node.js version? (For example Node.js 6.9.1 on Mac OS X)
   Node.js 10.6.0

3. Does the action work when you manually perform request against mailgun using curl (or other means)?
   Yes

4. Sample source code or steps to reproduce

(Write description of your issue here, stack traces from errors and code that reproduces the issue are helpful)

I am thinking how we can add tags to the emails?
I add the parameter o:tag to the data passed to send method.
However I can add only one tag while Mailgun supports up to 3 tags for one email.
Is it supported somehow? If not I will try to make a PR for this.

Thanks,
Damian.