Support automatic connecting on the same email addresses

Instead of not creating new users if ones with the same email addresses exist, I suggest it would be better to allow users to connect automatically in such a situation. I've made a simple demo [here](https://github.com/w1ndy/sociallogin/commit/8d2abb81676c2c422976ad695181db4108215482). 

If it's acceptable I can proceed to make it configurable and propose a pull request. Thanks!