add Leet open command to open current question in browser

[Rusteam]

Closed #54.

Can you please check if the changes make sense? I'll update the docs in the meantime.

I've tested on macos, it works.

[kawre]

Logic should go inside the open function because it's not related to the console

[Rusteam]

Can you point me to a specific file?

[Rusteam]

@kawre fixed all

[kawre]

Changed it back to os.execute since i didn't know that vim.cmd will log the command executed. Ideally it should use plenary jobs since they are non blocking, but for some reason they don't work well on windows.

Thank you for your contribution