Reform how we generate blog article summaries

From the updated Changelog:

* The `summary` method on articles is now HTML-aware, and can be provided with a different summary length or ellipsis string: `summary(1000, '&hellip;')`
* The `summary_generator` option now recieves the rendered article (without layout) instead of the template source.
* Set `summary_length` to false to always use the full article as a summary.