Support reading of the meta fields

[judgej]

Updating products is one thing, but access to custom meta fields is also very handy. This plugin should support the reading of this data too.

The WC wc-api/v2/orders API does support meta fields, but wc-api/v2/products does not. A similar format can be adopted. An example of an order meta field on an order line is:

"line_items": [
{
    "id": 106,
    "subtotal": "400.00",
    "subtotal_tax": "0.00",
    "total": "400.00",
    "total_tax": "0.00",
    "price": "200.00",
    "quantity": 2,
    "tax_class": null,
    "name": "Product B from Vendor B",
    "product_id": 12,
    "sku": "1234567890-2",
    "meta": 
[
        {
            "key": "shipping_line_id",
            "label": "Shipping_line_id",
            "value": "110"
        }
    ]
},

The "meta" array contains the fields, each having a key, a label and a value.

[judgej]

This format may be where the source of the question in issue #5 came from, and maybe the format should be adopted there too, so input and output fields are in the same format.

[judgej]

Completed. Return format is in the main README.md documentation.