Issue title: Need an option to clear messages in group
 Issue body: First thanks for the code.

I am looking for an option to clear messages in my groups. Can you help me on this?
 Comments: 
 Comment 0: First of all apologies for the late reply. I have added this feature, Kindly pull the repository or re-download the latest whatsapp.py

*Usage:*

```python
from whatsapp import WhatsApp

whatsapp = WhatsApp(100)
whatsapp.clear_chat("F1 Group")
````

Thank you and regards,
Visweswaran N
 Comment 1: @rajasekar-addobyte Kindly close the issue if it has solved your problem.
 Comment 2: Thanks for the update. It works. i used it to clear multiple groups

```
from whatsapp import WhatsApp
import time

whatsapp = WhatsApp(100)

participants = ['Group1', 'Group2']


for participant in participants:
    print("Sending: ", participant)
    time.sleep(1)
    try:
        whatsapp.clear_chat(participant.strip())
    except Exception as e:
        print(e)
       
    time.sleep(2)
    whatsapp.goto_main()
```


{'source': 'GithubIssues', 'repo_name_with_owner': 'VISWESWARAN1998/Simple-Yet-Hackable-WhatsApp-api', 'issue_id': 472753220, 'repo_stars': 541.0}