Refactor resource helpers

Split `resources_for` method into smaller chunk
for reusability
Use partials for local includes instead of `resource.render`
Exclude partials from resource list
Add `data_fields` parameter

Collect additional fields in resource's front matter data
Add dynamic filterable tree view menu

The menu is dynamically created from APP_CONTEXT embedded json.
It is filterable by keywords (title, tags etc) exposed in page metadata.
Initial work to support directory_indexes extension
Use default body margin/padding for standalone pages
Fix test
Merge pull request #4 from tyom/better-nav

Navigation impovements