add support for absolute path instead of .ftppass/env variables

This commit adds support for passing the absolute path to a JSON file instead of the default `.ftppass` or the environment variables

**Example:**

_/home/JohnDoe/sftp/my-json-file.json_
```
{
    "username": "your-username",
    "keyLocation": "/path/to/your/key",
    "password": "your-strong-password"
}
```

_Gruntfile.js_
```
"sftp-deploy": {
    build: {
       auth: {
           host: 'my-ftp.com',
           port: '5678',
           authKey: '/home/JohnDoe/sftp/my-json-file.json'
       }
      ...
    }
}
```