Device type to be used in path configuration

Added ability to add %%current_device%% to config when you need to create template inheritance from phone to desktop, if phone template is missing.

```
liip_theme:
    path_patterns:
        bundle_resource:
            - %%bundle_path%%/Resources/themes/themeName_%%current_device%%/%%template%%
            - %%bundle_path%%/Resources/themes/themeName/%%template%%
            - %%bundle_path%%/Resources/themes/_base_%%current_device%%/%%template%%
            - %%bundle_path%%/Resources/themes/_base/%%template%%
```
need to call `init` to be sure that `devices` isn't empty