Add support for the `data-src` attribute to img

[jaywhj]

Fixed issue #65

1. Added use of data-src where src is used. data-src is a custom attribute that complies with the HTML5 specification for data sources
2. Prioritize data-src over src, because data-src is usually the "real resource path", while src may be a placeholder or null value, e.g., when loading images lazily, src tends to be a placeholder, and data-src is the real image