gitignore: ignore downloaded media so an in-tree run can't commit content

running got in the repo root drops downloaded files next to the source;
torrents/control files were already ignored, add common media (mkv/mp4/
avi/mov/webm/iso/mp3/flac) so anime/ISO downloads don't show up as
untracked and can't be committed by accident.

.gitignore

@@ -7,3 +7,13 @@ ref/
 bin/
 .torrent.db*
 *.part
+
+# media downloaded by running got in the repo tree (never commit content)
+*.mkv
+*.mp4
+*.avi
+*.mov
+*.webm
+*.iso
+*.mp3
+*.flac