Add log event property enricher support

[Arkatufus]

Add enricher functionality, ILogEventEnricher enrichers can be appended at the end of the log message arguments like so:

Context.GetLogger().Info("Actor received {ID}", o, new PropertyEnricher(
    "custom-property", 
    new
    {
        name = "Custom", 
        data = counter
    }));

[Aaronontheweb]

Need to add a unit test for this I think

[Aaronontheweb]

LGTM but I'd like to see a unit test added to cover this

[Aaronontheweb]

Need to add a unit test for this I think

[Arkatufus]

Added unit test for both default and serilog message formatter

[Aaronontheweb]

LGTM