Ability to register the SerilogLoggerFactory

I needed to retrieve some configuration prior to registering the logger. `UseSerilog` is invoked before the `Startup` class. I sought to register the Factory manually but found out that is internal. I think that `SerilogLoggerFactory` is the main take away from this library and therefore exposing it would make it more flexible for those who don't want to use the extension methods.

Author: jonathansant

src/Serilog.AspNetCore/AspNetCore/SerilogLoggerFactory.cs

@@ -18,7 +18,7 @@
 
 namespace Serilog.AspNetCore
 {
-    class SerilogLoggerFactory : ILoggerFactory
+    public class SerilogLoggerFactory : ILoggerFactory
     {
         readonly SerilogLoggerProvider _provider;