Remove usage of token from environment in Nostrum.Api.Base

[jchristgit]

Find a way to pass in the bot token from other sources than the application
environment without requiring users to update all their API calls.

This will be more complicated than #545. I think that the only clean way to
implement this is to add the API token as a parameter to each call in
Nostrum.Api. I can't imagine a clean other way to do it.