Add speaker info to xml export

[loomchild]

Adds speaker slug, biography and other info to the xml export in a new persons section.

Alternative to #266

[loomchild]

I would like to also add the speaker photo, but I see it's a bit more complicated by looking at layouts/schedule/speaker.html. Could you share how the speaker object looks like in sample/export/pentabarf.yaml (as I don't have access to the actual export)?

I think speaker photo is not critical and can be complicated (considering resizing, etc.), so I can propose a separate PR for it.

[johanvdw]

I think speaker photo is not critical and can be complicated (considering resizing, etc.), so I can propose a separate PR for it.
I'm planning a change to the URL's generated for attachments and speaker images. It may simplify things so I would wait for that.