Allow fetching Mixpanel library from a different URL

[henrinormak]

What does this PR do? Allows configuring a custom URL to load Mixpanel.js from

Are there breaking changes in this PR? No.

Any background context you want to provide? Allows loading Mixpanel from a proxy-server

Is there parity with the server-side/android/iOS integration components (if applicable)? N/A

Does this require a new integration setting? If so, please explain how the new setting works There is a new option/setting - customLibraryUrl, which if specified is used as the source for the script tag used to load the library.

Links to helpful docs and other external resources N/A

[henrinormak]

Any updates? Or am I better off figuring out some alternative?