Added setting protobuf-custom-targets

The new setting can be used to add target (directory, src file extension) to the protoc generation, e.g. when using a protoc plugin. Custom directories are removed from clean and re-created from generation. Custom file extensions are used to identify new generated files, and return them as source files.
Improved management of custom targets

Setting protobuf-custom-targets has changed name to protobuf-generated-targets, and can be used to affect also java class generation. Back compatibility is retained
Update README.md
Merge pull request #19 from lsgro/master

Added setting protobuf-custom-targets