Configurable: Should source files be auto-attached as reference files?

If source files are converted by Okapi, currently the original source files are automatically attached to the task as reference files.

In the future this should depend on a Zf_configuration value. Default should be "attach them".

Be careful: The file conversion should be generic, meaning that in the future it should be possible to also integrate other file converters than Okapi. If the current programming makes it possible, thus the new config option should be generic and not Okapi specific.