Add PageList Plugin and routes.

* GET: cmd/list: show page list
* GET: sitemap.xml: show site map XML formatted
Add method to Page model for get priority fot site map.

Top layer: 1.0
Second layer: 0.8
Third layer and more depth: 0.6
Use priority
Add cmd/list.json route

Response page list formatted JSON.
Merge pull request #32 from hokuken-inc/feature-list-plugin

Add PageList Plugin and routes.