[#4,all][l]: first pass implementation of extensions registry with basic stuff working.

What is working:

* Basic jekyll site with CKAN theme working (copied from
  https://github.com/ckan/ckan.github.io and then cleaned up and stripped down)
  * fixes #3 - theme site
* Home page listing of all extensions based on okfnlabs.org/project page
  * fixes #2 - basic home page (though not really finished)
* One example extension (with frontmatter) in extensions/geo.md

What is not:

* Have not written all extensions to extensions/ directory - so not really listing extensions yet!
* Have not really modded the projects listing code so e.g. filter not yet set up for our case