Can not remove user from company #121

Version info

• intercom-go version: v2.0.0-20190319111853-1dbafb072bcd
• Go version: 1.12.5

Expected behavior

When saving a user with one or more companies which has Remove set to true, I expect the user to be removed from that company as per API documentation:

To remove a company from a user add the field remove with a value of true to the embedded company object and submit the user as a update request. You must also submit the id of the company.
https://developers.intercom.com/intercom-api-reference/reference#companies-and-users

Actual behavior

The SDK doesn't even send that attribute:
https://github.com/intercom/intercom-go/blob/v2/company.go#L35

Steps to reproduce

1. Try to save a user which has a company with Remove set to true
2. See how nothing happens