added isCommentsSupported method

extractor/src/main/java/org/schabi/newpipe/extractor/StreamingService.java

@@ -92,7 +92,8 @@ public abstract class StreamingService {
     public abstract PlaylistExtractor getPlaylistExtractor(ListLinkHandler urlIdHandler) throws ExtractionException;
     public abstract StreamExtractor getStreamExtractor(LinkHandler UIHFactory) throws ExtractionException;
     public abstract CommentsExtractor getCommentsExtractor(ListLinkHandler urlIdHandler) throws ExtractionException;
-
+    public abstract boolean isCommentsSupported();
+    
     public SearchExtractor getSearchExtractor(String query, List<String> contentFilter, String sortFilter, String contentCountry) throws ExtractionException {
         return getSearchExtractor(getSearchQHFactory().fromQuery(query, contentFilter, sortFilter), contentCountry);
     }

extractor/src/main/java/org/schabi/newpipe/extractor/services/soundcloud/SoundcloudService.java

@@ -102,13 +102,16 @@ public class SoundcloudService extends StreamingService {
 
 	@Override
 	public ListLinkHandlerFactory getCommentsLHFactory() {
-		// TODO Auto-generated method stub
 		return null;
 	}
 
 	@Override
 	public CommentsExtractor getCommentsExtractor(ListLinkHandler urlIdHandler) throws ExtractionException {
-		// TODO Auto-generated method stub
 		return null;
 	}
+
+    @Override
+    public boolean isCommentsSupported() {
+        return false;
+    }
 }

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeService.java

@@ -146,4 +146,9 @@ public class YoutubeService extends StreamingService {
 		return new YoutubeCommentsExtractor(this, urlIdHandler);
 	}
 
+    @Override
+    public boolean isCommentsSupported() {
+        return true;
+    }
+
 }