Basic source maps support

Right now I don't know how we could support source maps pointing to the exact line in js which generated a particular rule property or even a rule. Though we could at least point to the file by creating a babel plugin which will add the file name to `jss.createStyleSheet(styles, {fileName})` or `injectSheet(styles, {fileName})` calls. The function name should be customizable.

Then JSS can render a comment `/*# sourceMappingURL=` after the css string.