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 
is this still being used in this version of the formatMoney? It's also a little strange that it's being created outside on its own