Rules for default messages

[jamesmills]

@ostark

I'm still not keen on showing a price range (suggested number of trees to buy) by default.

If the extra.treeware.teaser is empty then we can use the default message with no tree count.

If the extra.treeware.teaser has a custom message use that.

If the extra.treeware.priceGroups is empty then don't show anything.

If the extra.treeware.priceGroups has a value then show it.

It's easy to use the README to explain to people the options they have to add a teaser and/or the priceGroups.

Thanks