New Pagecontext transformer

Here is an implementation of a more complete `PagecontextTransformer` that populates the data object with information related to the context of generation:

- `siteroot` : as before
- `pagename` : name of the page, without extension
- `breadcrumbs` : an array containing the list of directories of the current path

I also completed `template.twig` and added some example content to show how to use such information.

Tell me what you think !