Add selected_streams optional argument

[abcsds]

I'd suggest to add an optional argument to the read_xdf function that receives either a list of integers with the desired streams ids, or a list of strings with the stream names. What do you think is best? both?

[cbrnr]

Stream names do not need to be unique, so I think it should be a list of stream IDs.

[cbrnr]

BTW, the parameter is called select_streams in pyxdf.load_xdf and stream_ids in a wrapper function that I have written for MNELAB. Maybe it would be a good idea to reuse one of these names?