GetUser method should handle params

[Seraf]

Hello,

On auth0 API specs, there's some fields to pass through the params to return a subset of users : https://auth0.com/docs/api/management/v2#!/Users/get_users

The GetUser method should handle an optional params map to be able to filter the users returned

[xlr-8]

The GetUsers method already take a params variable: https://github.com/cycloidio/goat/blob/develop/auth0/users.go#L30-L32. However I don't think it is tested yet, so should be added.

[xlr-8]

I think indeed the BuildParamsURL function is incorrect, I think it should looks like something like this:

-   for _, val := range params {
-       params_url += "?" + val
+   for key, val := range params {
+       params_url += "?" + key + "=" + val