Write out unknown tags for MediaPlaylist too

Fixes https://github.com/rutgersc/m3u8-rs/issues/55

src/playlist.rs

@@ -764,6 +764,10 @@ impl MediaPlaylist {
         if let Some(ref start) = self.start {
             start.write_to(w)?;
         }
+        for unknown_tag in &self.unknown_tags {
+            writeln!(w, "{}", unknown_tag)?;
+        }
+
         for segment in &self.segments {
             segment.write_to(w)?;
         }