Add support of flags

[n10v]

The version is followed by the ID3v2 flags field, of which currently
four flags are used.

a - Unsynchronisation

 Bit 7 in the 'ID3v2 flags' indicates whether or not
 unsynchronisation is applied on all frames (see section 6.1 for
 details); a set bit indicates usage.

b - Extended header

 The second bit (bit 6) indicates whether or not the header is
 followed by an extended header. The extended header is described in
 section 3.2. A set bit indicates the presence of an extended
 header.

c - Experimental indicator

 The third bit (bit 5) is used as an 'experimental indicator'. This
 flag SHALL always be set when the tag is in an experimental stage.

d - Footer present

 Bit 4 indicates that a footer (section 3.4) is present at the very
 end of the tag. A set bit indicates the presence of a footer.

http://id3.org/id3v2.4.0-structure

[n10v]

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