Add flag to show cmd input/output

I personally found myself wanting to see exactly what commands where being run, and what their output was, using [clint](http://github.com/kennethreitz/clint). I was constantly repeating myself (print command, run command, print output and return code).

This lead to me abandoning use of envoy in favor of a pattern like [this](https://gist.github.com/rattrayalex/57bd40b2de7f8868d25f). Needless to say, copying this into every file was getting to be a chore, but it didn't seem like "enough" to warrant its own library. 

If this seems like a worthwhile addition, please take it. Otherwise, would love advice from others on how best to release. 

This is my first pull request to open source that I'm aware of, so please let me know if I'm deficient/out of line in any way here!