Support React Native

[steida]

This is just an idea since I'm not sure yet what is the best approach. The code for native version is very simple: https://github.com/niborb/react-native-gravatar/blob/master/index.js

Suggested React Native import syntax:

import Gravatar from 'react-gravatar/native';

[KyleAMathews]

I'd support a PR adding this. I don't use React Native right now but it makes a lot of sense to support it. It should use the same props though as the web version.

[steida]

The first step should be children as function. Then we can use RN specific elements.