[FEATURE] Simplify serialization path

[codebrain]

Since it is not possible to serialize directly into the StringBuilder, then you can simplify this method:

		protected override void RenderFormattedMessage(LogEventInfo logEventInfo, StringBuilder target)
                {
                        // blah blah
			var output = ecsEvent.Serialize();
                        target.Append(output);
                }

Into this:

		protected override string GetFormattedMessage(LogEventInfo logEvent)
		{
                        // blah blah
			var output = ecsEvent.Serialize();
                        return output;
		}

And remove the existing GetFormattedMessage

[snakefoot]

@codebrain I think this issue should be closed. I'm creating a PR that will make better use of the StringBuilder. See also #75

[russcam]

Thanks @snakefoot, closing this.