fix regression: allow to provide webpack devtool
fix regression: allow to provide webpack devtool

"devtool" is used in development.js and production.js but never defined. This commit fixes this.
Now you can provide devtools like this:

```
gg({
    "baseDir": __dirname,
    "developmentDevtool": "cheap-module-eval-source-map",
    "productionDevtool": "none"
}).setup(gulp);
```
goreutils/gore-gulp,zangrafx/gore-gulp