Add possibility to set own JsonSerializerSettings in configuration

This allows to pass own settings to be used for server-side rendering, for example the same you've got in your ASP.NET configuration:

```csharp
var settings = GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings;

ReactSiteConfiguration.Configuration.SetJsonSerializerSettings(settings);
```
Oh yeah, good point! I totally forgot about that.