Plot PeriodIndex data with steps
I would find it more natural for PeriodIndexed data to be plotted by default as a stepped line. A step better conveys the fact that the y-value one is plotting corresponds to that entire x-region, whereas a (linearly or otherwise) interpolated line suggests that one is using Timestamped data (since it suggests that the y-value is different at each point in time, which for periods is impossible to know).

It is, of course, possible to do this manually with `drawstyle=steps`, but this has the annoying problem that one of the Period "bins" will be missing (the one at the end). `steps-mid` is ugly, since it cuts off half of the steps at both edges.

It would be nice if the default would be changed to this `drawstyle`, but it would be even better if this style would be extended some way to fully handle Periods. Ideally, each Period step would be plotted between the edges of the Period and irregular Periods would be supported as well.
