Build does not contain templatetags package

On generating build files using `python3 setup.py sdist bdist_wheel`, the `build` folder does not contain all necessary packages.

```text
build/
├── bdist.linux-x86_64
└── lib
    └── django_mermaid
        ├── apps.py
        └── __init__.py
```

**Test on test.pypi.org before publishing**

PR:
 - #7 