Extract and abstract issue types.

At the moment, issue types seem a bit inflexible. Here are a few thoughts:

* Move them into a plugin. * Move their definition into config files and provide a set of examples to get you started. Configuration could include type name, how it is rendered in changelogs/ditz internal displays, etc. This also paves the way for new issue types--support requests entered from a web interface, for example. * Make existing operators check for the plugin's presence, not requiring it to be used.