Added directive to specify HTTP proxy credentials in config.

The inline directive http-proxy-user-pass can be used to
specify proxy credentials in config, e.g.:

http-proxy proxy.tld 3128 auto-nct
<http-proxy-user-pass>
foo
bar
</http-proxy-user-pass>

This usage is already supported by OpenVPN 3.

Signed-off-by: James Yonan <james@openvpn.net>