Add number-of-pages to locale by copying pages

As per @rmzelle's comment on https://bitbucket.org/fbennett/citeproc-js/issue/148/number-of-pages-label-does-not-render

I suppose it's more flexible to have `number-of-pages` defined independently of `page`

Edit: there's also a bit of an issue with how number-of-pages plurality is determined at least in citeproc-js. A single number appears to be interpreted as singular.