Wordpress-style <!-- more --> tag

This PR allows the user to make use of a wordpress-style `<!-- more -->` tag to denote the end of the article summary.  To use it, you must enable the setting
```
SUMMARY_END = '<!-- more -->'
```
This is superseded by the explicit setting of the summary in the article metadata, and
is entirely backward compatible: if `SUMMARY_END` is not specified, the previous behavior is preserved.