Add configurable minimum MP4 fragment duration and Matroska cluster duration

todo.txt

@@ -2,5 +2,5 @@
 - https://github.com/Vanilagy/mp4-muxer/issues/83 tell him it's possible now
 - is this fixed? https://github.com/Vanilagy/webm-muxer/issues/50
 - cross-track offset for streaming sources
-- configurable fragmented mp4 fragment size, like the mp4-muxer PR
-- textsubtitlesource, chunked piping
+- textsubtitlesource, chunked piping
+- More efficient MP3 loading when reading sequentially