[options] Accept quoted string across multiple lines (#9940)

Like:

    -f "
    bestvideo+bestaudio/
    best
    "

youtube_dl/options.py

@@ -26,9 +26,7 @@ def _readOptions(filename_bytes, default=[]):
         except IOError:
             return default  # silently skip if file is not present
         try:
-            res = []
-            for l in optionf:
-                res += compat_shlex_split(l, comments=True)
+            res = compat_shlex_split(optionf.read(), comments=True)
         finally:
             optionf.close()
         return res