[JSX] Support for boolean attributes shorthand

[youzi]

jsx supports both of these for "attribute:true":

var Hello = <Hello personal={true} />;

and

var Hello = <Hello personal />;

Should we support both as well (meaning we set undefined attributes vals to true)?

more info: https://github.com/yannickcr/eslint-plugin-react/blob/master/docs/rules/jsx-boolean-value.md

[youzi]

could be nice, since some linters (like sublime-babel) default to enforce the latter syntax