Added configuration of custom fastcgi_params [fixes #389]

I created a PR for [Fastcgi Params #389](https://github.com/jfryman/puppet-nginx/issues/389) and proposed the key "params" for adding custom fastcgi_params in the puppet configuration.

```puppet
nginx::resource::location { "root":
    ...
    ensure          => present,
    fastcgi         => "127.0.0.1:9000",
    params          => {
        'APP_ENV' => 'local',
    },
}
```