Add ability to underline every Nth line in --long

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 `N`th 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 ` where 0 would disable the feature completely, 1 would underline every line, 2 would underline every 2nd line, etc.
This would greatly improve accessibility.

Start by locating the --long output path and the command-line option parsing for lsd. Define how --underline N interacts with --long, including 0 disabling it and 1 underlining every line; done means the requested intervals are visibly underlined and the option is documented in the command's help.

Written by the indexing model from the issue text.