FR: Option to not collapse missing lines

[ilyagr]

With the same setup as #16, I'd like an option for MisMerge to show empty space for the missing lines instead of collapsing it to a line. So, all three of the windows would have 3 lines, but the missing ones would not have a line number and would be colored differently.

Collapsing missing lines can be very useful if there is a huge block, but can also be confusing, so I'd make it optional.