optional support n headlines, that alway visible at top of screen

[jaegdi]

Hi, at first: great job and i like moar. i'm unhappy, that my go skill is not so good, that i can implement that by myself an send a pull request.

I often have long output that has on the fist n lines a header. It would be nice, if moar can support this by an optional parameter like eg. -headlines=3. Then the fist 3 lines are always on the top of the screen and the other lines a scrolling (like in excel with fixed rows)

best regards