Ability to use proxy to download consul binary

[seppaleinen]

First of all thanks for a brilliant lib!

Working behind a proxy makes downloading the consul-binary impossible.
It would be nice to be able to pass some information, so that a proxy is configured.

Either by environment variables as with the CONSUL_BINARY_CDN, or by adding a constructor with the extra fields.

Thanks again!

[ruslansennov]

What about the system properties http.proxyHost and http.proxyPort?

[pszymczyk]

@ruslansennov if you are 100% sure that system proxy do the job we can add some manual to readme.md

[ruslansennov]

I'm not sure, but I'll check it out