Please add dynamic adapter loader by config

For example:

``` php
$config = [
'adapter' => '{adapterName}',
'host' => 'host',
'user' => 'user',
//etc...
];

/* @var Phalcon\Db\Adapter\Pdo\{AdapterName} $dbAdapter*/
$dbAdapter = Phalcon\Db\Adapter\Pdo\Factory::load($config);
```

This feature actual for phalcon components with multiple adapters (db, sessions, cache etc)
