Respect FFMpeg path passed via Environment Variable

MediaBrowser.Controller/Extensions/ConfigurationExtensions.cs

@@ -23,6 +23,11 @@ namespace MediaBrowser.Controller.Extensions
         /// </summary>
         public const string FfmpegAnalyzeDurationKey = "FFmpeg:analyzeduration";
 
+        /// <summary>
+        /// The key for the FFmpeg path option.
+        /// </summary>
+        public const string FfmpegPathKey = "ffmpeg";
+
         /// <summary>
         /// The key for a setting that indicates whether playlists should allow duplicate entries.
         /// </summary>