Add <Link replace> and <Redirect push>

[aaugustin]

These props allow selecting pushState and replaceState for navigating to
the next state.

Fix #3903.

[timdorr]

LGTM. Thanks for including the docs updates! 👍

[timdorr]

@mjackson or @ryanflorence any thoughts on this? I think it's fine for what it is.

[ericf]

I ended up implementing a custom <Navigate> component in my app to deal with this. It definitely feels like a missing gap for a common use case.

import {Component, PropTypes} from 'react';
import {router as routerShape} from 'react-router/PropTypes';

export default class Navigate extends Component {
  static contextTypes = {
    router: routerShape.isRequired,
  };

  static propTypes = {
    to: PropTypes.oneOfType([
      PropTypes.string,
      PropTypes.object
    ]).isRequired,
  };

  componentDidMount() {
    this.context.router.transitionTo(this.props.to);
  }

  render() {
    return null;
  }
}