Shared Clients

[CarsonF]

This allows using a different clients from the SDK. Fixes #5

To use:

<SplitProvider config={...}>

  <SplitClient splitKey="carson" trafficType="admin">
    <Split feature="foo">
      ...
    </Split>
  </SplitClient>

</SplitProvider>

If SplitClient is optional and if it isn't specified the default client will be used.
This works as you'd expect a React provider to work, so SplitClient can be nested and the closet parent client is used.