Store additional item fields as data- attributes

If additional item fields are defined on the DataSet or array being
passed to the timeline, store them as data- attributes on the content
element.
Define allowed dataAttributes as Timeline option

Read from an array of allowed dataAttributes at the timeline level and
write them as data- attributes on the item
Add data-attribute example
Update timeline options to include dataAttributes
Merge pull request #290 from dturkenk/data-attributes

Store additional item fields as data- attributes