Create a shell task

[KenSuenobu]

Create a task that can be easily used for invoking a shell command.

[ngbinh]

while at it, please consider https://github.com/brettwooldridge/NuProcess instead of ProcessBuilder

[KenSuenobu]

I will definitely review it. We've done some pretty awesome stuff with ProcessBuilder, and made sure that it works with cross-platform, which is the most important piece to this task. If this works well, I might end up using it. If you want, once the code is in place, I can assign you as a reviewer.

[KenSuenobu]

I've decided to forego using this library, as I want to keep things simple. I can leave it up to other developers to implement their own version of a process handler for the ShellTask.

[KenSuenobu]

Code finished.