Make curve_interval() work directly inside stat_lineribbon()

[mjskay]

this is more of a questioning thing... not sure I want to re-architect treatment of the point_interval argument to stat_lineribbon() to support this since it involves making more assumptions about the type of function that can be passed there (since the function must be able to do grouping itself instead of having the grouping be done by the stat prior to calling the function). Better option might be to just add an example where use of curve_interval() with the data argument to geom_lineribbon() is shown.