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.