Added whitelabeled attributes.

[dyatlov]

This allows to have IMG tags displaying correctly in resulting content.

Example:

    doc, err := readability.NewDocument(html)
    if err != nil {
        return
    }

    doc.WhitelistTags = []string{"div", "p", "img"}
    doc.WhitelistAttrs["img"] = []string{"src", "title", "alt"}
    fmt.Println( doc.Content() )