Fix a bug with taxonomy rendering.

- In `layouts/_default/taxonomy.html`, the `.Data` result does not provide the same information that `layouts/_default/terms.html` does for being able to identify the plural value of the term.
- This change adds `.Data.Singular` and `.Data.Plural` to provide similar capabilities.
- This *may* be incompatible with templates that check for `{{ if ne $taxonomy "Pages" }}` if the `page.Params` has either the singular or plural values as keys.

Note: alternatively, this could implemented by changing line 821:

```diff
- n.Data[t.singular] = t.pages
+ n.Data[t.plural] = t.pages
```

I think that the approach I’ve taken is more appropriate and similar to the results in `RenderListsOfTaxonomyTerms`.