Allow add extra attributes in playlist items.

[javiercornejo]

This feature allow to add extra attributes to items in a playlist. E.i: playlist.items.PlaylistItem[lastIndex].attributes.attributes['#EXT-X-MDSTRM-CUE-POINT-OUT'] = "DURATION=#{Number(segment.cueOut.duration).toFixed 4},#{segment.cueOut.id}"