Multiple stream synchronization

[neuromechanist]

Unfortunately, the MOBILAB plug-in is crashing while handling object attributes. I have identified the likely problem (sccn/mobilab#20) but it seems that the effort to fix the issue would be equal, if not more, to make load_xdf do the intended work.

Following the discussion with @arnodelorme (sccn/eeglab#810), I'll try to make this plug-in slightly extended so it can synchronize multiple streams and keep data within the EEG structure.