Truncate DataFrames centrally, rather than at one end
This came up in discussion on #5550 and #5594. The default plain & HTML repr now truncates DataFrames to the first max_rows rows and max_columns columns. At least for rows, truncating in the middle (so we show the first `max_rows//2` and the last `max_rows//2` rows) would be more consistent with the display of Series and numpy arrays.

This isn't a major change, but it's not completely trivial either, which is why I haven't done it already. I'll try to get round to it, but if someone else wants to work on it, go for it.
