Allow alternative devtools

[zangrafx]

Sometimes it is usefull to use other than built-in sourcemap options. For example in large projects with 70+ entry points devtool: source-map fails to complete due out of memory.

I've made pull request #52 with easiest way to do it, so you can use it like this:

gg({
  "baseDir": __dirname,
  "productionDevtool": "cheap-source-map"
}).setup(gulp);

or

gg({
  "baseDir": __dirname,
  "productionDevtool": "none"
}).setup(gulp);

[mcharytoniuk]

I released the patch in 0.11.8 version. I'll investigate the issue and check what can we do about 'out of memory' errors.

[zangrafx]

Thanks!