Update README.md
Remove _get_listening_port()

This was only used by the systemd path, which doesn't print
"Listening on http://...:$PORT" anymore.
Add ability to listen on different interfaces

This adds code to parse listener lines using the same syntax supported
by nginx[1]. This allows specifying ipv6 without having a dedicated
'ipv6' flag in the configuration file.

Parsing is very bare bones and not much checking is performed: only
basic tokenization is done and validation is performed by
getaddrinfo().

[1] http://nginx.org/en/docs/http/ngx_http_core_module.html#listen
lwan_status_*() formatting strings do not require '\n'

Newlines are added automatically.