Handle under/overlow bins more flexibly

[MoAly98]

At the moment the under and overflow bins are included in all histograms by default when the histgroams are created. The question of whether or not they should be included in the plots should be up to the user with a sensible default.

I believe by default mplhep.histplot will not show the the flow bins, but a simple bool can get them on the plot, so this should be simple enough to implement. Mplhep can show the bins, or it can sum them to the first and last bins, that should also be up to user.