The `among` branch in my git repository refactors IkiWiki::Render a
bit, then uses this refactoring to add a [[ikiwiki/directive/pagestats]]
parameter `among` that will only count links from pages that match a
given [[ikiwiki/PageSpec]]. From its documentation:

> The optional `among` parameter limits counting to pages that match a
> [[ikiwiki/PageSpec]]. For instance, to display a cloud of tags used on blog
> entries, you could use:
> 
>        \[[!pagestats pages="tags/*" among="blog/posts/*"]]
> 
> or to display a cloud of tags related to Linux, you could use:
> 
>        \[[!pagestats pages="tags/* and not tags/linux" among="tagged(linux)"]]

Suggestions for a better parameter name are welcome. --[[smcv]]