Add option for Dropzone without context (bring-your-own non-HTML5 context)

[smallfx]

I wanted to use this library (because react-dnd was interfering with react-dropzone) and it would have been perfect ... except I ran into an error with multiple HTML5Contexts :)

So this PR will allow someone to use their own global DragDropContext() via import { DropzoneWithoutContext }

Additionally, they (obviously) could use some other backend than HTML5Context if they need to for whatever reason.

Example

import * as React from 'react';
import { DragDropContext } from 'react-dnd';
import HTML5Backend from 'react-dnd-html5-backend';
import { DropTarget, Target } from 'react-dnd-dropzone';

const HTML5DropContext = DragDropContext(HTML5Backend)(({ children }) => (
  <div>{children}</div>
));
const Dropzone = DropTarget(Target);

const App = () => (
  <HTML5DropContext>
    <Dropzone
      onDrop={files => console.log(files)}
      render={({ canDrop, isOver }) => (
        <div>
          <pre>{JSON.stringify({ canDrop, isOver }, null, 2)}</pre>
        </div>
      )}
    />
  </HTML5DropContext>
);

[evenchange4]

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Built with commit d59c584

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[codecov]

Codecov Report

Merging #56 into master will not change coverage.
The diff coverage is 62.5%.

@@         Coverage Diff          @@
##           master   #56   +/-   ##
====================================
  Coverage      70%   70%           
====================================
  Files           1     3    +2     
  Lines          10    10           
  Branches        2     2           
====================================
  Hits            7     7           
  Misses          2     2           
  Partials        1     1

Impacted Files	Coverage Δ
src/Target.js	100% <100%> (ø)
src/Dropzone.js	100% <100%> (+30%)	⬆️
src/DropTarget.js	40% <40%> (ø)

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[evenchange4]

@smallfx Amazing! Thanks for the PR.

It is better to expose all the small pieces of components (Target & DragDropContext) so that consumers can compose their own Dropzone component.

[evenchange4]

Thanks! Released v1.1.3 with your changes. 🙌

[smallfx]

@evenchange4 Awesome! Thanks for the help! 😄