add the missing file back for the automatic_alt_tag feature

Just noticed this working off master for the meta page asset paths --- I believe this is coming from the removal of the automatic_alt_tags.rb file in b813db397f51a35bddb80b7afafea4a00ccc51ba

```
/Users/username/code/middleman/middleman-core/lib/middleman-core/core_extensions.rb:94:in `require': cannot load such file -- middleman-more/extensions/automatic_alt_tags (LoadError)
```

Assuming the specs are good -- should be good to go from here.