Merge strategies support

[mutcianm]

With current implementation if there are multiple files mapped to the same path, the latter would just overwrite the former.
This can be worked around by using additionalFileMappings setting, but having a more generic solution would be better.
In sbt-assembly plugin this is solved by introducing a MergeStrategy concept which takes care of resolving conflicts.

Implementing similar concept would prove useful, although coherency with IDEA readable artifacts must still be obeyed

[mutcianm]

This seems to be impossible to map onto IDEA's artifact building mechanism which is required to be able to develop IJ plugins from within IJ itself.