Support centering search results

[araaha]

In less, when using the -j (--jump-target) option, we can center search results by specifying a line on the screen where the "target" line is to be positioned. In this case, -j.5 leads to less jumping on search results in the middle of the screen. See image:

whereas in moar, the jump target is always at the top. Ideally, a similar feature is implemented in moar, or at least allowing the jump target to be at the top, middle, or bottom of the screen.