introduce user-defined configuration

This change introduces configuration via .json files, environment
variables, and command-line parameters.  Defaults are baked into the
config.js' module.
dynamically configure listening port

Dynamically allow the configuration of the blpapi-http server port to
listen on.  By default the port listens on 80 unless '--port' or the
'port' configuration is specified otherwise.
config: pass the config path prefixes as a parameter
Use command line arg "--cfgdir" to change the cfg dir