Define encoding for frames

[rstemmer]

Allow the user to define the encoding of the text inside frames. Currently only UTF-16 will be used (as defined by the ID3v2.3.0 standard).
With ID3v2.4.0 UTF-8 is allowed and should be possible to use.
Further more very old player would benefit from ISO8859-1 encoded frames :)

!! it is important to update the version number when a ID3v2.4.0-Only encoding is used!

• --set-encoding $FRAMEID $ENCODING or --fix-encoding … to only change the encoding byte of the frame, not the text itself (in case the encoding byte is wrong)
• --encoding $ENCODING to specify the encoding of new created frames
• --force-encoding $ENCODING to write all tags encoded as specified

[rstemmer]

--encoding $ENCODING implemented for id3edit 2.0.0