Support multi-proxy configuration

I have a particular use case that the current grunt-connect-proxy cannot satisfy: in order to run my karma functional tests, I need to proxy localhost against a deployed test instance. I'd like to do this from the same gruntfile that goes along with the main project, but grunt-connect-proxy only supports one _proxies_ -block. In development mode, I don't want to proxy the root of my server, but when running functional tests with karma, everything should be proxied to the test server.

This pull request adds support for multi-task style configuration for proxies as well.