Expose stdin and stdout

[vadimdemedes]

Since stdin and stdout are customizable, all components should have access to them via context to avoid confusion. This is also useful for testing and stubbing those streams.

Before:

componentDidMount() {
  process.stdin.on('keypress', ...);
}

After:

componentDidMount() {
  this.context.stdin.on('keypress', ...);
}

Maybe those should just be set directly on this, not this.context, haven't decided about this yet.

[vadimdemedes]

Fixed in Ink 2.