How to update device token after analytics is initialized?

[ggirotto]

In this PR https://github.com/segmentio/analytics_flutter/pull/73 it seems that you guys added a new property to Configuration class which is token, but what happens if the device token is only acquired after the analytics was already initialized? Should we create a new instance of the analytics to replace the old now or use the solution proposed on this issue https://github.com/segmentio/analytics_flutter/issues/29?