Add support for the `compat` key.

**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.