Running generators inside slice should infer --slice SLICE_NAME

Right now, you can only us generators from the main folder, but it'd be a nice feature to have the following be equivalent:

```
# In main project folder
hanami generate action home.index --slice admin
```

```
# in slices/admin/
hanami generate action home.index
```

Similarly, we should let generating from the `app/` folder work the same as it does from the main folder (since app/ is the default, it doesn't need to be any different).