Add automatic ERP markers

[ranyabel]

"extract ERPs live within the OpenBCI software. The user could declare the epoch length (or select a preset for common components), baseline correction period, event marker, etc. and watch the ERP form live in an ERP window. Users could make some cool biofeedback ERP games/paradigms! It could also serve as a tool for learning about ERP methodology-- being able to see in real-time how more and more trials average out noise/unrelated neural activity and reveal neural signatures associated with specific events would be very cool!"