MongoTarget/GetConnectionString method .net standart support

[kenannur]

GetConnectionString method now supports for .net standart.

[snakefoot]

Think it is a much better idea to make use of NLog Layout, and then just use ${configsetting}. Then you can skip the dependency-hell.

Example NLog.config:

<target xsi:type="Mongo" connectionString="${configsetting:item=ConnectionStrings.MongoDb}" />

Example appsettings.json:

{
  "ConnectionStrings": {
    "MongoDb": "mongodb://localhost/Logging"
  }
}

See also: https://github.com/NLog/NLog/wiki/ConfigSetting-Layout-Renderer