Update Min/Max values when Auto is triggered For Pointcloud plugin

[jgassaway]

In RVIZ, when coloring pointclouds by intensity, when you click "Autocompute Intensity" (the equivalent of mapviz's "Use Auto Max/Min"), the values for min/max intensity will automatically update to the max/min of the current pointcloud. These values can then be manually scaled by disabling that option and modifying the min or max values.
In mapviz, these values are not updated, so you must manually check the scale of your data by experimentation, or prior knowledge of the max/min bounds.

It would be helpful to update the max/min values displayed in the pointcloud2 plugin when "Use Auto Max/Min" is toggled.