[mixpanel] Expose additional initialization options

[nselikoff]

There are certain use cases where the default initialization options for the mixpanel client won't be sufficient: for example, implementing mixpanel in the EU, which requires a different api_host.

It would be great to allow users to specify these in the environment config and pass them through to mixpanel.init

Looks like #229 and/or #244 might address this issue.