Prevent overriding default path patterns with user configs

The goal of this PR is to prevent overriding default paths patterns with user configs.

config.yml
```
liip_theme:
    themes: ['web', 'tablet', 'phone']
    active_theme: 'tablet'
    path_patterns:
        bundle_resource:
            - %%bundle_path%%/Resources/themes/web/%%template%%
        bundle_resource_dir:
            - %%dir%%/themes/web/%%bundle_name%%/%%template%%
```

Dir structure:
```

├── Acme
│   ├── DemoBundle
│   │   ├── Resources
│   │   │   ├── themes
│   │   │   │   ├── tablet
│   │   │   │   │   └── index.html.twig
│   │   │   │   ├── web
│   │   │   │   │   └── index.html.twig
│   │   │   │   │   └── inner.html.twig
```

**Before:** Default paths would be overridden and all templates will load from `web` theme, but should from `tablet`
**After:** User paths will be appended to array