Update Shopify.formatMoney function

Fixes https://github.com/Shopify/shopify-themes/issues/286

- Check to see if Shopify.formatMoney is defined.
- If not, define it.

@carolineschnapp @mpiotrowicz @stevebosworth 
@carolineschnapp I'm removing this as it's not called anywhere. It's been too long since I included it so I can't remember, but do you know where/when this function would be used elsewhere (not necessarily Timber-related)?