add optionally options with root_path

Another implementation of #1 with tests. Adds the ability to pass optional options object with the root_path. Root path is used for all imports as the base path.

This implementation behaves like dcodeIO/ProtoBuf.js and doesn't conflict with the original libraries for Java, C++ etc.

Use the protobuf options to pass the information about root_path as it is dealt in #1 doesn't sound like clean solution.