refactor file.go, add archive & pagination

- there is a method to write articles & pages (less duplication).
- there are config booleans for show archive, tags & categories. In case
  that some of those is false in the configuration, then the pages are
  not going to be created.
  Note:  Some work is needed on the ifs inside the base template.
- there is a new design for the index page that is going to show all the
  article and a button to leave a comment.
- pagination was added.

TODO:

- I need to rethink about the config to show tags, archive & categories
  on the templates. Check case #17.

Fixes #1
Merge pull request #18 from agonzalezro/pagination

refactor file.go, add archive & pagination