Allow param to set ajax cart method

Read the `method` parameter, and if set, override the ajax cart method. Only valid methods are:
- `drawer`
- `modal`
- `flip`

[Demo flip](https://timber-demo.myshopify.com/products/true-flatlock-mens-tee-1?method=flip)
[Demo drawer](https://timber-demo.myshopify.com/products/true-flatlock-mens-tee-1?method=drawer)
[Demo modal](https://timber-demo.myshopify.com/products/true-flatlock-mens-tee-1?method=modal)

Navigating to a new page will remove the parameter and revert to the default `drawer`.

Fixes #158 

@Shopify/fed @carolineschnapp