closes https://github.com/TryGhost/Team/issues/545
The price helper requires an object with amount & currency properties to
work correctly. This updates the @price data object to expose these.
In order to maintain backward compatibility with using the @price data
as primitive number values, we add a valueOf method which returns the
legacy dollar amount value.
This means you can use {{price @price.monthly}} OR
{{@price.monthly}} - the second of which will output the dollar
amount.
A new theme fixture was added to test both usages of the @price data
