Add generic image parser

Image processor for common items was introduced in https://github.com/zytedata/zyte-common-items/pull/99 It could benefit from some sort of generic image parser:
```python
value = _handle_selectorlist(value)
if isinstance(value, (Selector, HtmlElement)):
    images = extract_images(value)
    return [Image(url=url) for url in images]
```