Compatibility with create-react-app 2.0 built-in CSS modules

[matteocng]

create-react-app 2.0, currently in alpha, supports CSS modules out of the box without ejecting, they just have to end with .module.css.

Since we are currently requiring .css files (SEE: ./src/styles), we know that this module will still not work, in CSS Modules mode (when cssModules: true is passed to FlagIconFactory), withcreate-react-app 2.0 ... unless the user ejected and modified the configuration of the CSS loaders.

We should add a section to the documentation explaining how to do that - tweaking the regex of the CSS modules loaders, after ejecting, to load .css files inside flag-icon-css and react-flag-icon-css as CSS modules, even though they don't have the correct extension.