Refactor tn-cli for better readability and wider usage

[THUzxj]

Is your feature request related to a problem? Please describe.

I want to interact with tinode with Python. tn-cli provides a python client implementation, but it is hard to read and use because:

• the single tn-cli.py file is too long
• the cli function and the python function is coupled

Describe the solution you'd like

I want to help to refactor the single file program to python modules, decoupling the cli functions and the python sdk functions.

Describe alternatives you've considered

None.

Additional context

None.

[or-else]

I suppose it's not a bad idea to split CLI from Tinode methods. If you decide to do it, please follow these two suggestions:

1. All PRs should be against devel branch.
2. Please do gradual refactoring, i.e. in small chunks so it would be easier to review and make changes.