Facade module for starting adapter-specific Repos in supervision trees.

Supports both single and multi-connection usage:

# Single connection
NexBase.init(url: "postgres://localhost/mydb")
children = [{NexBase.Repo, []}]

# Multiple connections
main = NexBase.init(url: "postgres://localhost/main")
analytics = NexBase.init(url: "postgres://analytics/db")
children = [{NexBase.Repo, main}, {NexBase.Repo, analytics}]