Add support for 3rd party service.xconfigs

3rd party service.xconfigs are located in %AppDataPath%/services
(e.g. %APPDATA%/OBS/services in non-portable mode) and have the same
format as a single service definition in services.xconfig without the id
field, e.g.:

"My Service" : {
  servers : {
    Primary : "rtmp://live.example.org/live"
    Secondary : "rtmp://live.example.com/live"
  }
  recommended : {
    "max bitrate" : 1337
  }
}