Add support for choosing listening host for Flask

[lstep]

If the server is on a remote machine, allows to set a listening ip/hostname, otherwise, only runs on 127.0.0.1.
Set it by default to 127.0.0.1 for security

[AndrewVeee]

Thanks, this is great, I really appreciate the contribution!

I did notice one issue - the config fails to load because the IP needs to be quoted, eg:
server_host = "127.0.0.1"

I hope I don't regret choosing toml - it's supposed to be a simpler config format, but it's not used much.

[AndrewVeee]

Actually, no big deal, I merged this and then updated the config.

Thanks again!