Ability to change product-grid-item snippet width with variable

Set `grid_item_width` before including `snippets/product-grid-item.liquid` to change its width. The variable should use the same classes as the grid.

Example:
```
{% assign grid_item_width = 'large--one-quarter medium--one-half' %}
{% include 'product-grid-item' %}
```

A default is set in the snippet if a size is not set, so this will not break any current code.