Generate static value column

Sometimes, I need to generate CSV format data that has columns with static values. You could do that by supplying  block that returns static value like:
```ruby
comma :static_column do
  publisher 'Publisher' do 'Foo, Inc.' end
end
```
But, it would be clearer if we have special field name to indicate producing static data column like:
```ruby
comma :static_column do
  __static_column__ 'Publisher' do 'Foo, Inc.' end
end