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,
NVIM_LISTEN_ADDRESS