Add proxy support in kpm

- Add proxy option to "kpm restore" subcommand
- "kpm restore" uses global proxy (http_proxy env var) by default

parent #215 
@davidfowl  , will we have `Environment.SetEnvironmentVariale()` in CoreCLR? Passing the proxy address as an argument is not desirable, it's a long calling chain from here to the actual method that needs this address.