Add option to define how current version is stored.

[arenoir]

Implement feature to make it easy to use nginx to serve the index page. Nginx config example at ember-cli-deploy/ember-cli-deploy#91

This pr adds a config option copyToKey. When defined the activate function copies the value of the specified version to the copyToKey value record. This makes it easy to serve the index page from nginx without having to do a primary lookup.

Example.

//my-app/config/deploy.js
module.exports = {
  production: {
    store: {
      type: 'redis',
      host: 'localhost',
      port: 6379,
      copyToKey: 'current-copy'
    },
}

nginx config

#this sets a variable $app_version from a url parameter version or defaults to 'current-copy'
map $arg_version $app_version {
    default "current-copy";
    ~^\w+ $arg_version;
}
upstream redisbackend{
    server 127.0.0.1:6379;
}
server {
    location / {
        set $redis_key my-app:$app_version;
        redis_pass redisbackend;
        default_type text/html;
    }
}

[arenoir]

Taken care of in ember-cli-deploy-redis ember-cli-deploy/ember-cli-deploy-redis#43