Adds variable support for plugins and maxBuffer size

This PR allows you to set variables when configuring a plugin.

```
plugins: [
    '/local/path/to/plugin',
    'http://example.com/path/to/plugin.git',
    {
        plugin: 'http://example.com/path/to/plugin.git',
        variables: [{
            name: 'API_KEY',
            value: '0118 999 881 999 119 7253'
       }]
    }
],
```

It also allows you to set the maxBuffer size for child processes as some builds go over the default buffer size on iOS.

I had problem running the tests so I've not completed work there unfortunately.