user-configurable expandableRowHeaderColDef

Related to issue #1990.

This allows the user to configure the expandableRowHeaderColDef by adding a $scope.gridOptions.expandableRowHeaderColDef to js files.
The var at the top applies to the comma-separated list. defaultExpandableRowHeaderColDef, userInjectedExpandableRowHeaderColDef, and finalExpandableRowHeaderColDef are all declared with the var keyword