Add in authentication

Hey Andrew!
I went ahead and added some auth features.

The basic idea is that you can now do...

```bash
nserver --auth p@ssw0rd
```

Then when the user hits `localhost:8000`, they'll be directed to a login screen.

The login screen will prompt for a password.

Once the correct password has been entered, the duration of the session will not require any further authentication and will work as normal.

Let me know if you love or hate this, or have any additional questions.

Thanks!