Can't specify a helper for facet fields.

[tpendragon]

There doesn't seem to be a way to specify how a facet field is displayed, via something like a helper parameter.

[cbeer]

We let index and show field configuration provide a :helper_method to use when rendering the value. For consistency, we should do the same for facet fields, I think.

I believe one work-around is to override facet_display_value in your own helper