Add a "host" option to openfisca-serve

[bonjourmauko]

Imported from openfisca/openfisca-web-api#132 by @Morendil.

When using the openfisca serve script, I can specify a port but not a host to bind to; the script defaults to localhost which is appropriate only in a narrow range of conditions. It will for instance prevent usage from a Docker container, a throwaway virtual machine on a cloud provider, or that home NAS that you use as a server for tinkering with stuff.

Possibly a better default is 0.0.0.0, a pseudo-host which will result in the server socket binding on every available interface. Alternately, a config option --host to override localhost is perfectly suitable.