Dash.jl should automatically set port if corresponding PORT environment variable exists

[rpkyle]

Currently, Dash.jl does not attempt to detect whether a PORT environment variable has been set before launching the server, so using a statement such as

run_server(app, "0.0.0.0")`

when deploying the app may fail to bind to the correct port. This may cause issues with Dash Enterprise or Heroku unless a workaround is used, e.g.

port = parse(Int64, ENV["PORT"])

run_server(app, "0.0.0.0", port)