Expose background server to custom interface

[nvictus]

The background servir server currently binds to 127.0.0.1 with no way to change it. PR

When higlass-python is running on a jupyter notebook hosted on a remote kernel, tilesets hosted by the user on that kernel will not be available because the server socket is not bound to a public-facing IP.

The straightforward solution that sidesteps this issue to use hg.server.enable_proxy(), but:

1. It is currently not documented.
2. It may not work in all cases, in which case exposing the server to a public interface 0.0.0.0 (assuming the port is accessible) is an alternative.