Fix #1220 -- allow reading_time in Read more links [WIP]

This is #1220.

This is not finished yet.  We need to implement `READ_MORE_LINK` as a TranslatableSetting, so it would work for multilingual blogs, too.

Suggestions welcome.  We might also want a new default value for the setting.  And, if we want to be more hardcore, make my unfulfilled dream come true (#412, #574) and create `{INDEX,RSS}_READ_MORE_LINK`.

About the paragraph count thing, it should not be awfully hard to implement.  That’s about enough:

```py
lxml_document.xpath('count(//p)')
```

Further commits and improvements upcoming.