This commit adds a new dcos task exec subcommand to the DC/OS CLI. Using dcos task exec, one can launch arbitrary commands inside a task's container in a DC/OS cluster. This includes commands that simply print output to the screen (e.g. ls -la), run interactively (e.g. cat), or take full control of the terminal (e.g. vim or bash). Author attribution: Steven Locke @StevenLocke