custom jinja2 blocks as metadata

This allows to define jinja2 blocks in the text and have them accessible as metadata.

Example
```
date: 2013-01-10
title: title

{%block summary %}
Here goes the summary

{% endblock summary %}

more text
{% block conclusion %}
Nothing

{% endblock conclusion %}
```

Now one can use e.g. `article.summary` and `article.conclusion` and the theme templates.

Fixes #358

Knwown Caveat: does not play well with the Markdown 'attr_list' extension (which gets loaded by 'extra' per default). Just take care that the markdown parsers does not mess up the block definition (newlines help)

I will update the docs etc... after the discussion of the proposed solution is over.