options: add option for nested chapters
matroska: factor out chapter parsing

This is in preparation for supporting nested chapters.
matroska: parse out nested subchapters
matroska: add TODO item for hidden editions
Support nested chapters

Since ChapterAtom can be nested within itself, a magic value of '' for
the elid is used to not add the element to the lists a second time.
parsing
mplayer: print out chapters
WIPWIPWIP