Add option for injecting styles at top of head

Currently, the function output by this package uses `document.head.appendChild(css)`. I'm using `react-toastify` and would love an option to pass to `inject-styles()` for inserting the css with `document.head.insertBefore(css, head.firstChild)`, because I have custom style overrides in a stylesheet above the injected styles that aren't taking precedence due to the order.