Add support for continuation on <orderedlist>

[sandersk]

Let's add support for the attribute continuation="continues" on <orderedlist> elements. This should be transformed into a start attribute on the output HTML5 <ol> element, with the proper numerical value.

[sarahs]

Needs more testing so I'm leaving this issue open, but PR #59 is a start.

[sandersk]

Awesome, this is great! Thanks, Sarah.