Configuration and cleanup of orphaned files added

Hello Matthias,

I'm Fèlix, the one you talked to at StackOverflow regarding the cleanup and placement of generated files for this plugin of yours. I've just added some code to it in order to gain two new features:

* The output dir will be read from a `_config.yml` parameter named `ditaa_output_directory`. If not specified, it will get the default value hardcoded inside the plugin.
* Cleanup of orphaned ditaa-generated files. It looks for `ditaa-xxx.png` files and compare to the list of generated in this build. The program deletes all the extra files.

I've also updated the README.md to document those changes.

I hope that's ok for you!