Mix Cmd on Umbrella none recursive

[Mike Binns]

Currently if you run mix cmd on an umbrella app, it runs the cmd on each child app. You can't currently use mix cmd on an umbrella to run the command once at the top level. I was thinking about submitting a PR with a `cmdu` task that runs the specified command at the umbrella level.

For example, I attempted the below mix alias and it resulted in the second part of the alias running once for each of the child apps instead of once at the umbrella level:

coverage_report: ["coveralls.html --umbrella", "cmd open cover/excoveralls.html"]

-Mike Binns

GitHub: TheFirstAvenger