Override Preview Scroll Index Limit

Currently, if text shown in the preview pane exceeds the height of the window, you're able the scroll down it. The issue is that the scrolling exceeds the actual text contents and will continue until the very last line of text is at the very top of the scroll preview.

Now, this makes sense when the preview pane has its dynamic wrapping enabled. But pre-formatted text is passed in and Fzf knows the exact number of lines it will take up, it's a strange limitation.

Nothing complicated is needed, just a simple variable for power users would be appreciated — something that overrides the maximum index of scrolling in the preview pane.