Allow HOST and PORT config via ENV variables

Hello,

Instead of:

```
def set_trace(addr="127.0.0.1", port=4444):
```

It would be nice to have:

```
set_trace(addr=None, port=None):
    if addr is None:
        addr = os.environ.get('REMOTE_RIPDB_HOST', '127.0.0.1')
    if port is None:
        port = os.environ.get('REMOTE_RIPDB_PORT, 4444)
```

What do you think about it ?