avconv: move audio_sample_fmt to options context.

Also document it and replace undocumented and inconsistent
'-sample_fmt list' syntax with -sample_fmts.

cmdutils.c

@@ -717,6 +717,15 @@ void show_pix_fmts(void)
     }
 }
 
+int show_sample_fmts(const char *opt, const char *arg)
+{
+    int i;
+    char fmt_str[128];
+    for (i = -1; i < AV_SAMPLE_FMT_NB; i++)
+        printf("%s\n", av_get_sample_fmt_string(fmt_str, sizeof(fmt_str), i));
+    return 0;
+}
+
 int read_yesno(void)
 {
     int c = getchar();