Lack of `__init__.py` file in ./tailor/common/filters

**Is your feature request related to a problem? Please describe.**
When installing from Pypi (tailor-nlp 0.1.0), the following bug will appear:
```
ModuleNotFoundError: No module named 'tailor.common.filters'
```
It turns out that there is no `__init__.py` file under path `./tailor/common/filters`, so this directory would be ignored and totally lost when installing the package.

**Describe the solution you'd like**
Add a blank `./tailor/common/filters/__init__.py`

**Describe alternatives you've considered**
N/A

**Additional context**
N/A