Add option to filter chat by date

## Concept idea

### When loading chat

```python
from whatstk import WhatsAppChat
chat = WhatsAppChat.from_txt('file.txt', date_min='2020-01-01', date_max='2020-01-21')
```

### When analysing chat

```python
from whatstk import WhatsAppChat
chat = WhatsAppChat.from_txt('file.txt')
chat = chat.filter_dates(date_min='2020-01-01', date_max='2020-01-21')
```
