Add ability to underline every Nth line in --long

[vercas]

Reading the output with --long can be difficult at times, since the rows are quite wide.
A very easy way of making the output more readable is to underline every Nth line.
For my personal preference, every 5th line is perfect, but I think it would be best to have this as an option like --underline <integer> where 0 would disable the feature completely, 1 would underline every line, 2 would underline every 2nd line, etc.
This would greatly improve accessibility.

[Equationzhao]

Do you mean underline or split line?

Underline

Split line (I think this is better)

[vercas]

@Equationzhao Underline - the first one. It's enough to improve readability, in my opinion. No need to use entire rows just for a line.
(also nothing stops you from adding both, if you've already gone through the trouble of implementing this)