text: Title: Only output should be kept for keep_output and init_cell
  Body: Currently, `nbstripout` ignores `keep_output` and `init_cell` cells entirely. However, this means that lots of stuff beyond the output stays, such as `execution_count`. I think keeping this does not meet the design goals, and it can cause Git conflicts.
  
  Feature request: Treat these two types of cells normally, except keep the output.