Loads the specified app file into a Feersum server.
In both cases, if --pre-fork=N is specified, that many worker processes are
used to serve requests. See Feersum::Runner for details.
If in native mode (when running feersum --native), currently only a
--listen parameter is accepted. By default, the server will listen on
localhost port 5000.
When running in PSGI mode (non-native), Plack::Runner is used. See that
module for documentation and defaults.