Give focus_crawl a chance to access page body before discarding it

For site-specific crawlers, it's fair enough to use `focus_crawl` like this:

```
anemone.focus_crawl do |page|
  if page.doc
    page.doc.search('.//a[@href]').map { |a| URI.parse(a[:href]) }
  else
    page.links
  end
end
```

However when using the `discard_page_bodies` option, `page.doc` is `nil` by the time we enter this block. In this pull request I've moved until after `focus_crawl` has been called.