Make already evaluated columns accessible to preprocessing block

[cblock]

Make already evaluated columns accessible to preprocessing block

[tatey]

Hey,

Thanks for the feedback. I had not thought of this use case before.

At the time each column's preprocessor is evaluated, all of the columns are known, but not necessarily evaluated. We'd need to do something like build a lookup table which evaluates the column the first time, and returns a cached value thereafter. This type of behaviour could lead to circular dependencies if we weren't careful, so the README would need to caution.

The columns are evaluated in Conformist::Builder and the preprocessor is an attribute of Conformist::Column.

I'm too busy to dive into it now, but might work on it at RailsCamp.