[post_stats] Python 3 compatibility

In Python 3, `filter()` returns an iterator instead of a list, which causes an error on line 42: "object of type 'filter' has no len()". Using a list comprehension fixes this, and is probably a little more readable.
Properly encode author in comments
Allow restructuredtext comments.
Take into account some review comments.

- Automatically exclude comments path from pages and articles processing.
- Add the file extension back onto the ID.
- Update the documentation.
Update docs about the comment system dir.
Replaces the comment id with slug

 - The slug is more robust since it does not contain spaces, ...
 - Compute the slug from the file name (including the extension, to prevent collision)
 - Possibility to use a custom slug