org-opml-template: put org-opml Github URL in <generator> element

Technically, this violates the spec because it's not in a
namespace. But Dave Winer's Fargo outliner includes [1] custom <head>
elements, so I don't think it's the biggest sin in the world.

Plus, the spec does say to ignore elements you don't understand.

[1] https://dl.dropboxusercontent.com/s/ge8ql807di0rzkt/kimParker.opml