Added typescript typings file

[vasilevich]

This pull request contains a simple index.d.ts file and package.json configuration to point to it.
it allows you to seamlessly use this library inside your react-redux project while also utilizing typescript typings.

I think it will also allow IDEs such as VSCODE to hint better while using this library.
Here is a typical usage example in a typical typescript project:

import * as React from 'react';
import {Provider} from 'react-redux';
import store from 'reducers';
import RouterPage from 'containers/RouterPage';
import I18n from 'redux-i18n';

/**
 * Entrance of app
 * The program starts here
 * first we grab our app store
 * and then we place our main app route which is our pages routes switch
 */

export class App extends React.Component<object, object> {
  render() {
    return (
      <Provider store={store}>
        <I18n translations={{
          es: {
            '_hello_': 'Traduce este texto',
            'Hello {n}!': 'Hola {n}!',
          },
        }} initialLang="es">
          <RouterPage/>
        </I18n>
      </Provider>
    );
  }
}

Which might look similar to normal JS. however without the typings, it would not let you import I18n as easily within typescript projects.

and then, in any child component within <RouterPage/>

example typescript usage:

import * as React from 'react';
import {Container, Image, Grid} from 'semantic-ui-react';
import {ControlProps} from 'containers/ControlPage';
import FullScreenButton from 'components/Control/Components/Navigation/Components/MenuManager/Components/MenuLink/FullScreenButton';
import {fullHeightStyle} from 'utils/Style';
import {PropTypes} from 'react';
import {IGetTranslateFunctionResponse} from 'redux-i18n';

export default class Main extends React.Component<ControlProps, never> {

  static contextTypes = {
    t: PropTypes.func.isRequired,
  };
  context: { t: IGetTranslateFunctionResponse };
  constructor(props: ControlProps) {
    super(props);
  }
  render() {
    return (
      <Container style={fullHeightStyle} fluid>
        <Grid>
          <Grid.Column width={14}>
            <Image centered src="https://react.semantic-ui.com/assets/images/wireframe/image.png"/>
            <div>{this.context.t('_hello_')}</div>
          </Grid.Column>
          <Grid.Column width={1}>
            <FullScreenButton {...this.props} />
          </Grid.Column>
        </Grid>
      </Container>
    );
  }
}

thank you for your time!
cheers.

[francescarpi]

Thanks!