Add placeholder expression {+} to preview window

[vredesbyyrd]

Really enjoying Skim, thanks for sharing this awesome tool.

One option I made use of from fzf allowed for the following...

fzf doc explains the preview window placeholder expression, +, with the following:

A placeholder expression starting with + flag will be replaced to the space-separated
list of the selected lines (or the current line if no selection was made) individually
quoted.

For my particular use case I am writing a small utility for the buku bookmark manager. I would like to use sk to display a list of tags, and use the preview window to indicate all selected (toggled) lines, as shown in the fzf screenshot above.

If its possible to port this option it would be a great addition to sk!

Thanks for your time.

[vredesbyyrd]

Thanks for implementing this feature, works great :)