Allow specifying '{n}' as the OFFSET in the preview-window flag

[jaydee-coder]

This feature can be useful in scrolling to the selected line in the preview window.

For example, it can be used to use fzf as a quick filtering tool for large files while keeping an eye on the selected line as it shows up in the original file.

Example usage:

man fzf | tee /tmp/tmp.log | fzf \
  --reverse \
  --exact \
  --query 'position' \
  --preview 'bat --force-colorization /tmp/tmp.log --highlight-line $(({n}+1))' \
  --preview-window '+{n}/2'

[junegunn]

Merged, thanks!