Add a helper to generate tags and categories.

This helper code was inspired by
https://gist.github.com/telemachus/2228812. See
http://ithaca.arpinum.org/2012/03/30/adding-categories.html for
technical details. I made some modifications:
1. I don't use nanoc's builtin tag support.
2. Attributes include tags and categories, however, in general, any
array like attributes in article's metadata is well supported.
3. It is recommend to associated one article with only one category, and
multiple tags. However, this is not a strict rule.