Allow to set log file prefix and ITextFormatter.

closes BedeGaming#39

src/Serilog.Sinks.RollingFileAlternate/LoggerConfigurationExtensions.cs

@@ -116,6 +116,39 @@ public static LoggerConfiguration RollingFileAlternate(
             return configuration.Sink(sink, minimumLevel);
         }
 
+        /// <summary>
+        /// Creates an alternative implementation of the rolling file sink with
+        /// an overload to pass an ITextFormatter.
+        /// </summary>
+        /// <param name="configuration"><see cref="LoggerSinkConfiguration"/></param>
+        /// <param name="formatter">Formatter to control how events are rendered into the file. To control
+        /// plain text formatting, use the overload that accepts an output template instead.</param>
+        /// <param name="logDirectory">The names of the directory to be logged</param>
+        /// <param name="logFilePrefix">The prefix for the log file name.</param>
+        /// <param name="minimumLevel">Minimum <see cref="LogLevel"/></param>
+        /// <param name="fileSizeLimitBytes">The size files should grow up to (default 2MB)</param>
+        /// <param name="retainedFileCountLimit">The maximum number of log files that will be retained,
+        /// including the current log file. The default is null which is unlimited.</param>
+        /// <returns></returns>
+        public static LoggerConfiguration RollingFileAlternate(
+            this LoggerSinkConfiguration configuration,
+            ITextFormatter formatter,
+            string logDirectory,
+            string logFilePrefix,
+            LogEventLevel minimumLevel = LevelAlias.Minimum,
+            long? fileSizeLimitBytes = null,
+            int? retainedFileCountLimit = null)
+        {
+            if (configuration == null)
+            {
+                throw new ArgumentNullException("configuration");
+            }
+
+            var sink = new AlternateRollingFileSink(logDirectory, formatter, fileSizeLimitBytes ?? TwoMegabytes,
+                retainedFileCountLimit, logFilePrefix: logFilePrefix);
+            return configuration.Sink(sink, minimumLevel);
+        }
+
         /// <summary>
         /// Creates an hourly rolling file sink that rolls files every hour.
         /// </summary>