Add configuration file support

[mb-emag]

Right now doxysphinx has to be called for each and every doxygen doxyfile with nearly the same arguments over and over again.
While this can be automated easily a config file will have other benefits too.

We should provide the possibility to provide a repo wide config file at a specific location (e.g. repo root) and have all arguments configured there - then a call to doxysphinx (without any arguments) will be sufficient.