Allows posts to be named as they were in Jekyll

* Forks wintersmith-tag from node_modules into plugins
* Uses article iteration from https://github.com/acdlite/andrewphilipclark.com/blob/master/plugins/paginator.coffee in
  paginator.coffee and wintersmith-tag.coffee
* Removes wintersmith-tag from package.json
* Renames seiko post back to its original Jekyll name (date-title.md)