Allow users to reuse Http when building the Client

[arqunis]

This adds a new construction method to the ClientBuilder, new_with_http, that will allow users to reuse their instances of the Http client.

This can be useful in the case when a user creates a Http instance to retrieve their bot's user id for the Configuration::on_mention method, and then wish to pass that instance to the ClientBuilder to bypass unnecessary creation of a second Http instance.