Fixes #149 -- Add levelSwitch to config

[jotabe-net]

The sink configuration extension methods missed a "levelSwitch" parameter. I included it, as an optional parameter, in all the extension methods.

I deployed locally a package with this update and tested the behavior, and everything looks fine.

I did the testing on a full .NET Framework Web API app, with a JSON config file, and it works. Should work exactly in the same way in .NET Core. Relevant part of the configuration:

"Serilog": {
  "Using": [ "Serilog.Sinks.File", "Serilog.Sinks.ApplicationInsights" ],
    "LevelSwitches": {
      "$aiLevelSwitch": "Information"
    },
    "WriteTo": [
      {
        "Name": "ApplicationInsights",
        "Args": {
          "telemetryConverter": "Serilog.Sinks.ApplicationInsights.Sinks.ApplicationInsights.TelemetryConverters.TraceTelemetryConverter, Serilog.Sinks.ApplicationInsights",
          "levelSwitch": "$aiLevelSwitch"
        }
      }
    ]

[nblumhardt]

Thanks, LGTM 👍