Add "sh" config option, and a Windows keybind

This adds an "sh" config option that controls whether or not the command is passed to /bin/sh. Also, it adds a Windows keybind. As an additional :gift:, I've added a "configuration options" section to the readme that details each setting and what they do.
Just to make it consistent. The default value should probably be set in the return statement. This will also keep `sh` from being undefined in all cases except `.atom-build.json`-one.