Consider re-adding GraphViz / UML support

[Danny Ben Shitrit]

The GraphViz dot integration was removed in #39

The self-proposed approach would be to do one of these:

1. Extract all diagram handling to a new gem. This will require adding plugin functionality to madness.
2. Convert the current dot handling code to a more generic, configurable handler with some magic like { from: 'dot', to: 'png', with: 'command to run' }
3. Enable the diagramming only in the docker version. An attempt at this was made, but due to Vagrant file watching issues, it is not working well (at least not in a Vagrant-based environment).

[Danny Ben Shitrit]

Unlikely to happen, no demand.