[feature request] add column header

[bruceauyeung]

when we run ps without any args, its output includes column header like this:

# ps
  PID TTY          TIME CMD
22671 pts/0    00:00:00 bash
23572 pts/0    00:00:00 ps

currently gops does not have column header, those newcomer users may not know meanings of 1st and 2nd column

[yashLadha]

I would like to work on this issue @bruceauyeung