connect() method can use NEOVIM_LISTEN_ADDRESS 

Hi @tarruda 

Thanks for this library, i just improve the possibility to use the NEOVIM_LISTEN_ADDRESS in connect() method if address is empty, with this improvement we can directly execute python script in neovim like (neovim set NEOVIM_LISTEN_ADDRESS if not exists)
```
:! python myscript.py
```

Regards,
Avoid exclamation marks in user-facing messages. Suggestion:

> NVIM_LISTEN_ADDRESS not set

This matches long-standing patterns, for example if you `cd -` in a new bash shell, this is the error message:

> OLDPWD not set