Configuration and cleanup of orphaned files added

[ghost]

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!

[matze]

Hi Fèlix, that's very nice, thanks for the PR.

I have one remark and maybe you have an opinion on that: You are now suppressing the ditaa output which is probably the right thing to do on success. However when something goes wrong, it would be very helpful to see the cause of the problem. Apart from that it looks good for merging.

[ghost]

Take a look now. I've added a simple ditaa_debug_mode to control that specific.

[matze]

That's great! Thanks for your submission.