Add a before_redirect hook to alter redirects

[rmccue]

This allows implementing things like browser-compatible redirect handling:

$hooks->register( 'requests-requests.before_redirect', function ( $location, $headers, $data, &$options, $original ) {
    // Browser compat
    if ( $original->status_code === 301 || $original->status_code === 302 ) {
        $options['type'] = Requests::GET;
    }
});

Fixes #201, see #178.