implement the Mixpanel integration

[jipiboily]

[GabLeRoux]

https://mixpanel.com/

[jonashelgemo]

@jipiboily: I take it this should still be implemented? If so, I can start looking at it soon. Looks like there are several projects that implement a golang-client (like this one: https://github.com/dukex/mixpanel), but no official one. I prefer to just to an integration against the http-endpoints directly (documented here: https://mixpanel.com/help/reference/http) to avoid being at the mercy of some library author should the mixpanel api change. Agreed?