making sure that execution of scripts defined in build.json->exec are calling back into the main execution path

If you're using `require('shifter').add()` to add new files into the queue to be executed, and you use `exec`, shifter will exit without continuing the execution of the queue. This patch fix that. Here is an example file:

```
{
    "name": "stencil",
    "exec": ["./resources/shifter_exec.js"],
    "builds": {
        "stencil-css": {
            "cssfiles": [
                "dist/base.css",
                "dist/grid.css",
                "dist/grid-responsive.css",
                "dist/table.css",
                "dist/skins.css",
                "dist/components.css",
                "dist/helpers.css",
                "dist/declarations.css",
                "dist/fx.css"
            ],
            "assets": true
        }
    },
    "shifter": {
        "coverage": false,
        "lint": false
    }
}
```