Add preview border style 'line'

It draws a single line between the preview window and the rest of the
interface. i.e. automatically choose between 'left', 'right', 'top', and
'bottom' depending on the position of the preview window.

CHANGELOG.md

@@ -43,6 +43,7 @@ CHANGELOG
         - `change-header-label`
         - `transform-header-label`
 - Added `--preview-border[=STYLE]` as short for `--preview-window=border[-STYLE]`
+- Added new preview border style `line` which draws a single separator line between the preview window and the rest of the interface
 - You can specify `border-native` to `--tmux` so that native tmux border is used instead of `--border`. This can be useful if you start a different program from inside the popup.
   ```sh
   fzf --tmux border-native --bind 'enter:execute:less {}'