Add an append(QByteArray) method to the ExtractionResult

    This way plugins can choose to return the data in utf8 or as a QString,
    and the clients can either just let the standard QString::fromUtf8
    function do its magic, or implement some special handling if they wish.

    This speeds up the PlainTextExtractor quite a bit (60msec vs 8.3msec)

    Unfortunately, this meant discarding the extraction of WordCount from
    the Plain Text extractor. Though considering the speed difference, I
    think it is worth it.