[Proposal] SASL authentication in MemcachedConnector

This PR added SASL authentication support in MemcachedConnector.

I deployed Laravel application on Heroku. This application use Memcachier for session stored. (Memcachier is a memcached service.)

Memcachier is authenticated with SASL, but Laravel's MemcachedConnector don't support SASL Auth.

I made this PR.

## Usage

You add memcached options and sasl auth data in `cache.php`.

`memcached_options` key is memcached options.These are set by `Memcached#setOptions()`

`memcached_sasl` key is memcached sasl auth data.This section has two keys.`username` is a sasl username and `password` is a sasl password.

ex) `app/config/cache.php`
```php
return [
    'driver' => 'memcached',
    'memcached' => [
        ['host' => 'memcached_host', 'port' => 11211, 'weight' => 100],
    ],
    'memcached_options' => [
        Memcached::OPT_BINARY_PROTOCOL => true,
    ],
    'memcached_sasl' => [
        'username' => 'sasl_user',
        'password' => 'sasl_pass',
    ],
];
```