Add SourceContext to log routines

Hi there!

Your fantastic library is spamming my application logs 😆 Would you **accept a PR** that adds `.ForContext` where applicable so that the log levels for your namespace can be overridden like:

```json
  "Serilog": {
    "MinimumLevel": {
      "Default": "Information",
      "Override": {
        "Abot2.Crawler": "Warning"
      }
    }
  }
```

Cheers