background_run: Internal helper function to run a `plumber` instance on specific
host and port.
Description
Internal helper function to run a `plumber` instance on specific
host and port.
Usage
background_run(server, port = 8000, host = "127.0.0.1")
Arguments
- server
an instance of `plumber` class
- port
numeric port to pass to `server` instance
- host
character host value for `server` instance