Adds converter modules and sorters

Converters are now the result of combining multiple modules, and running them in the selected sequence. I hope to be able to completely remove converters by replacing them with templates that simply run some modules in a sequence, before running FFmpeg. Then I want to be able to parse a command into a template, so users have full control of what a template does.

Sorters each sort a list of streams after a single criteria. They can be chained in order to create complex chained sorters.

src/main/java/net/knarcraft/ffmpegconverter/streams/SubtitleStream.java

@@ -61,8 +61,8 @@ public class SubtitleStream extends AbstractStream implements StreamObject {
     }
 
     @Override
-    public @NotNull StreamType getStreamType() {
-        return StreamType.SUBTITLE;
+    public char streamTypeCharacter() {
+        return 's';
     }
 
 }