EyeTracker: `command` help section

[marcobarilari]

from Remi #74

Eyelink('Command', 'link_sample_data = LEFT,RIGHT,GAZE,AREA');
Eyelink('command', 'link_event_data = GAZE,GAZERES,HREF,AREA,VELOCITY');
Eyelink('command', 'link_event_filter = LEFT,RIGHT,FIXATION,BLINK,SACCADE,BUTTON');

We need to dig into the help section to see if that means we can set which eye to follow.