Allow overriding the API URIs

[ChaosFreak]

The Mixpanel support team have asked me to use the new "beta" version of the API to resolve some performance issues. However, since the base API URIs are coded in string literals, it's not possible to use an alternate URI. I've created this branch to allow you to override any of the three API URIs by passing them as part of the config parameters when creating a new instance.

[keolo]

Thanks for this PR! This seems like a rare edge case and I worry that it's going to create more confusion than benefit. I'm going to leave it open for now to see if anyone else wants this feature.

[eugeneius]

Mixpanel now offers EU data residency with separate API URLs, which makes needing to change them less of an edge case:

https://developer.mixpanel.com/reference/overview

Standard Server: api.mixpanel.com EU Residency Server: api-eu.mixpanel.com

[keolo]

Thanks guys, this has been merged into 5.1.0.