Prepend siteurl (or relative_url) to {tag} and {category} links

I *think*, though am not sure, that the [unit tests](https://github.com/danielrichman/pelican/compare/cateogry_tag_siteurls?expand=1#diff-a6e75eddaf0b1d483ecdfcac11dad813L215) for `{tag}`/`|tag|`/etc. links are testing incorrect behaviour.

This patch puts site-url in front of intra-site links to tags or categories.