add proxy parameter support to cli

[sureshdsk]

adding add proxy support to cli interface would be very useful.

example

gplaycli -s com.google.android.youtube --proxy proxy1

Yes. I know I can set environment variable export HTTP_PROXY="http://proxy1" and use gplaycli.
But, In my case I've to use a rotate proxy servers. If the cli support proxy parameter, I can handle the proxy location logic outside and provide proxy url input to gplaycli directly. I can create a PR for the same if you agree to adding proxy parameter to the cli.