Restore prepended and appended whitespace removal

Per the _current_ redoflac documentation:
  A benefit of this operation is it's ability to remove any extra spacing
  before and after tag values. For example, let's say you have a FLAC file
  with a TITLE tag of (minus the single quotes):

  '    The Great Escape    '
  After retagging, the TITLE tag would look like (minus the single quotes):

  'The Great Escape'

This commit restores that functionality (did it ever exist?).

All tag values are now explicitly read in and stripped of prepended/appended
whitespace (spaces, tabs etc) before being re-applied to a FLAC file.
Fix broken handling of TRACKNUMBER/TRACKTOTAL tags

Regarding previous commit: 60080632924ae4ca65a8d584dae78bf0960b5a24

Strip the leading and trailing whitespace from the TRACKNUMBER and
TRACKTOTAL tags, BEFORE we try and prepend the zero (if specified in the
user configuration).