Allow registering custom widgets

[n1k0]

Self-explanatory dream API:

import Form from "react-jsonschema-form";

const MyComponent = (props) => {
  // ....
};

const schema = {
  type: "string"
};

const widgets = {
  myComponent: MyComponent
};

const uiSchema = {
  "ui:widget": "myComponent"
};

render((<Form 
  schema={schema} 
  uiSchema={uiSchema} 
  widgets={widgets}
/>), document.querySelector("main"))

Context: as uiSchema should be defined using JSON (and no necessarily JavaScript), we need to expose a way to register a custom component and associate it with a string unique identifier we can later reference from the JSON definition.

/cc @leplatrem

[n1k0]

Basic PoC in #67

[n1k0]

Fixed by #67.