Better user config

• You can now use environment variables anywhere in the configuration file
• You can now use environment variables without defining a default value (you can now use ${var} or ${var:default})
• You can now define nearly all proxy options. See http-proxy-middleware for a list of available options:

proxies:
  - /api: http://localhost:8080
  - path: /complex/route
    target: ${REDIRECT_TARGET:http://google.nl}
    prependPath: false
    ws: false
    pathRewrite:
      '^/old/path' : '/newPath'