[DRAFT] Add tags/categories/archives pages plugins

This is a _start discussion_ pull request (real one will be re-factored and recreated as a separate one).

These plugins adds "dynamically" generated pages for archives (list posts by years), tags and categories. Usage example can be found on my blog (https://github.com/ixti/ixti.github.com/):

* Configuration:
  * https://github.com/ixti/ixti.github.com/blob/source/_config.yml#L24
  * https://github.com/ixti/ixti.github.com/blob/source/_layouts/archive.html
* Output:
  * http://ixti.net/archives/2013.html
  * http://ixti.net/tags/jekyll.html
  * http://ixti.net/categories/software.html

Follows up of #867.