I've just found this comment from mojombo:
The latest on master now has Plugin support. Look at lib/jekyll/converters for examples of how they're done. Also, any *.rb files in a _plugins directory will be loaded so that you can create custom plugins of your own.
I've had a look /lib/jekyll/converters but could not understand how they were supposed to work.
Can anyone explain this to me ?
Thanks a lot.