mangle values for specific columns without deleting and re-adding.

[codeho]

Hi,

i was wondering if it would be possible to add some sort of
column_replace(:column_name { |resource| some_helper_function(resource.title) } )

to replace the value of a column.

this would be extremely helpful if one only wants to 'mangle' a few columns.
This way one wouldn't have to do all columns manually.

[randym]

This is being handled here - #9