fixes #674 - Support converting subtitles to webvtt

MediaBrowser.Controller/MediaEncoding/ISubtitleEncoder.cs

@@ -1,5 +1,4 @@
-using System;
-using System.IO;
+using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -7,9 +6,16 @@ namespace MediaBrowser.Controller.MediaEncoding
 {
     public interface ISubtitleEncoder
     {
-        Task<Stream> ConvertTextSubtitle(String stream, 
+        Task<Stream> ConvertSubtitles(
+            Stream stream, 
             string inputFormat, 
             string outputFormat,
             CancellationToken cancellationToken);
+
+        Task<Stream> GetSubtitles(string itemId, 
+            string mediaSourceId,
+            int subtitleStreamIndex,
+            string outputFormat,
+            CancellationToken cancellationToken);
     }
 }