Convert camelCase prop names to alllowercase

[alexcjohnson]

Regular HTML attributes use alllowercase, but React converts multi-word attributes to camelCase. There are a few exceptions: class turns into className, for turns into htmlFor. In dcc we're thinking of converting everything to snake_case plotly/dash-core-components#958 but in html we don't fully document these properties, just the prop names and basic description. The most natural place for users to look for the full documentation is however not React's docs but rather HTML reference sites like MDN so it feels like alllowercase would be more useful, and closer to the snake_case preferred by Python. We would still use snake for class_name and html_for.