v3 - add ability to pre-define devserver host and port in config.js

[Andy Fuchs]

right now the default parcel devserver host/ports are used, which are:

• localhost:1234
• 127.0.0.1:1337

To allow easier debugging on multiple devices simultaneously, allow binding to a custom host/port, eg.

"devServer": {
   "dev": {
       "host": "0.0.0.0",
       "port": 9876
   },
   "build": {
       "host": "0.0.0.0",
       "port": 9877
   }
 },