Add arguments to avoid hard-coding of protobuf_path and data_root_path

[francescomilano172]

Would it be possible to add two arguments to avoid hard-coding the path to the protobuf file and the type of dataset ('train' or 'val')? Something like francescomilano172@0a18291 (forked repo)? I find this handy when generating data automatically from a script.

[ntonci]

I guess you can open a PR here for this

[francescomilano172]

Ok done, I wasn't sure if I could do it

[margaritaG]

Fixed in #8.