Add format type B for outtmpl to treat the value as bytes

This is useful to limit the filename to a certain number of bytes rather than characters
Closes #1003

test/test_YoutubeDL.py

@@ -649,6 +649,7 @@ def test_add_extra_info(self):
         'title2': '%PATH%',
         'title3': 'foo/bar\\test',
         'title4': 'foo "bar" test',
+        'title5': 'áéí',
         'timestamp': 1618488000,
         'duration': 100000,
         'playlist_index': 1,
@@ -767,6 +768,7 @@ def expect_same_infodict(out):
         test('%(ext)l', 'mp4')
         test('%(formats.:.id) 15l', '  id1, id2, id3')
         test('%(formats)j', (json.dumps(FORMATS), sanitize(json.dumps(FORMATS))))
+        test('%(title5).3B', 'á')
         if compat_os_name == 'nt':
             test('%(title4)q', ('"foo \\"bar\\" test"', "'foo _'bar_' test'"))
         else: