Set subtype for Shopify alternate templates

Shopify allows to create [alternative templates](http://docs.shopify.com/themes/theme-templates/alternate-templates) by specifying an additional custom extension:

> An alternate product template will contain the word 'product' in it. It will be called something like `product.shirts.liquid`

But the HTML code in these templates would not be highlighted in Vim, only Liquid tags. I suggest to set the `html` subtype for Liquid files in the Shopify template folders.