Allow passing a gradient component (breaking change)

[dan-lee]

Hey @catalinmiron,

My goal with this PR is that one is able to pass any LinearGradient component to react-native-css-gradient.
This introduces a breaking change, as the default export is now is a creator function and no imports are done by this library itself:

import createCssGradient from "react-native-css-gradient";
import LinearGradient from "react-native-linear-gradient";
// or from expo:
// import { LinearGradient } from "expo-linear-gradient";

// create Gradient with gradient component.
const Gradient = createCssGradient(LinearGradient);

const App = () => (
  <Gradient
    // ...props
  />
);

I also took the freedom to make some minor tweaks:

• Add and apply prettier for code formatting
• Add types for typescript in index.d.ts
• Restructure and add install notes to README

Please tell me your thoughts :)