Add support for the `compat` key.

[DJDuque]

Is your feature request related to a problem? Please describe. The first thing that comes to mind is setting the width of a bar plot. Without the \pgfplotsset{compat=1.7} (or newer) the width of a bar is assumed to be in pt; it is much more convenient to work in units of the axis (need compat 1.7).

Describe the feature you'd like It would be useful to be able to set the compatibility for specific environments.

I don't know if something like this would only make sense in a tikzpicture environment. Or should this be set by plot, or by axis? Maybe just by picture is enough. Still not sure about this.