MINOR: httpclient/cli: implement a simple client over the CLI

MINOR: httpclient/cli: implement a simple client over the CLI

This commit implements an HTTP Client over the CLI, this was made as
working example for the HTTP Client API.

It usable over the CLI by specifying a method and an URL:

    echo "httpclient GET http://127.0.0.1:8000/demo.file" | socat /tmp/haproxy.sock -

Only IP addresses are accessibles since the API does not allow to
resolve addresses yet.