Feature: Add __main__.py for using with python -m

[glensc]

This allows easily run the program from source tree after installing dependencies:

$ pip install -e .
$ python -m subliminal --help

Adopted from:

• https://github.com/Taxel/PlexTraktSync/blob/458498a921ae25e0eb1193403a1637d2ac2555c2/plextraktsync/__main__.py

[getzze]

Thanks for the PR! It looks good to me, just need to fix ruff warnings:

• ruff automatically gets rid of print statements, just add this line as the first line of the file.

# ruff: noqa: T201

• you need to add a docstring to the file.

[glensc]

what does it mean, gets rid of print statements, what should one use then?

[getzze]

It is to remove print statement that have been added for debugging and forgot to remove them.
It can be quite useful :)

If you really need a print (like here), you can add an exception to ruff.