Standardising user parameters

[nigelsim]

I'm submitting a ...

• bug report

• feature request

• question about the decisions made in the repository

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem
  When specifying user properties each provider uses its own properties for email, first name, and last name. e.g., hubspot uses email while MixPanel uses $email.

• What is the expected behavior?
  It would be good to have a common structure for these properties, and then be able to map core properties to the provider specific property names.

• What is the motivation / use case for changing the behavior?
  We currently use GA, MixPanel and Hub Spot. It would be good to be able to have all of these map correctly without duplicating properties or backend data pushes.