add direct download support for channels, groups, and topics

[owbryd]

Summary

This PR adds support for downloading all media and messages from a Telegram channel, group, or topic directly using the --chat flag, without requiring a prior message export step.

What gets downloaded

• Media files — all attachments (photos, videos, documents, etc.)
• Text messages — saved as an .html file for easy reading

New flags

Flag	Description
--chat <id>	Target channel or group ID
--topic <id>	Target a specific topic within a group
--msg-start <id>	Start of message range (optional)
--msg-end <id>	End of message range (optional)

Usage & Examples

Download everything from a channel or group:

tdl dl --chat <chat_id>

# Example
tdl dl --chat 8493874389

Download everything from a specific topic:

tdl dl --chat <chat_id> --topic <topic_id>

# Example
tdl dl --chat 8493874389 --topic 9383

Optional: limit by message range:

tdl dl --chat <chat_id> --msg-start <start_id> --msg-end <end_id>

# Example (messages 100 to 500)
tdl dl --chat 8493874389 --msg-start 100 --msg-end 500