0.3.0b1 Fix `render_kw` ignored bug, use `field.__call__` instead of widget for render

What's Changed

• Fix #6 render_kw ignored issue by @fangpenlin in #7

Full Changelog: 0.2.3...0.3.0b1

While all the tests passed, not sure if there's unwanted side effects of using field.__call__, making a beta release first.