Proxy support for checks

[denisdefreyne]

@majkinetor wrote in nanoc/nanoc#698:

w3c validators and external links do not work behind the proxy.

For w3c validators this did it for me:

results = ::W3CValidators::MarkupValidator.new(:proxy_host=>'1.1.1.1', :proxy_port => 8080).validate_file(filename)

Similar is true for the request_url_once for elinks check.

I guess simple usage of environment variable http_proxy and friends instead of no proxy would suffice.