Add https proxy support

Adding https support to vmc command.
#66 added only the support for http_proxy and https support is missing.
vmc-lib already has https support by https://github.com/cloudfoundry/vmc-lib/pull/11 .

I wonder whether supporting `--http_proxy` and `--https_proxy` arguments by 'vmc' command is necessary or not. Supporting HTTP_PROXY and HTTPS_PROXY environment variables may be enough to set proxy servers.

BTW, do you have a plan to cherry-pack my pull requests on cf and cfoundry repository? I can send other pull requests to the repositories if needed.