Readd txt autodetection

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit e3147fe7767b0a98a472931c3fd3fddb534837c7)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/input/subtitles.c

@@ -61,7 +61,7 @@ static const char const sub_exts[][6] = {
     "idx", "sub",  "srt",
     "ssa", "ass",  "smi",
     "utf", "utf8", "utf-8",
-    "rt",   "aqt",
+    "rt",   "aqt", "txt",
     "usf", "jss",  "cdg",
     "psb", "mpsub","mpl2",
     "pjs", "dks",