Add support for page-specific look

[goosys]

I’ve added support for changing the look per page.

For example, like in the sample app, you can use a time_ago_in_words look only on the index page.

ATTRIBUTE_TYPES = {
  created_at: Field::DateTime.with_options(look: {index: :relative})
}

index	show
relative look	default look

What do you think?

[goosys]

@pablobm @nickcharlton
Rebased. Please take a look when you have a chance.