Add jsxhint checker

[JSXHint](https://github.com/strml/jsxhint) is a minimal wrapper around JSHint for linting JSX syntax, which is inline markup-in-JS used by [React](http://facebook.github.io/react/docs/getting-started.html). Since it wraps JSHint and has the same command-line options, this file is mostly a copy-paste of JSHint support.