Add support for plugins: Phase III

[oubiwann]

I'd like to write a dragon plugin that adds Flickr images to blog posts (of a given size, arrangement, and link back to Flickr). There's a ticket for this here:

• Come up with an alternative to the Flickr image widget forgotten-roads/blog#82

In order to do this, though, I need to inject a task into the normal content-parsing workflow, before markdown is converted to HTML.

In order to that, I need Dragon to be event-driven (see #1). With that in place, adding plugin support should be pretty straight-forward.

Phase I Tasks (Milestone 0.5.0):

• See Add support for plugins: Phase I #2

Phase II Tasks:

• See Add support for plugins: Phase II #43

Phase III Tasks:

• Identify other areas of Dragon that can be converted to using this new infrastructure
• Document how plugins need to be configured in order to
  • define a specific function that gives all its subscribers
  • define and publish pre- and post- events for every function that will operate on dragon data (so that other plugins may determine if they need to go before or after another plugin
  • listen for the right events
  • take actions at the right times
  • publish their own events
  • suggest plugin code layout:
    • name.plugin.main?
    • name.plugin.events?
    • name.plugin.subscribers?