Added ability to directly configure minification params

minify param in build section of ship.config.js can now have three possible values:

1. true — will enable default minification using UglifyJS. Default params are:

   {
       compress: {
           warnings: true
       }
   }

2. false — minification for production builds will be turned off.

3. object — configuration that will be explicitly passed to webpack.optimize.UglifyJsPlugin constructor.

   For example if you need to disable function and class names minification you can do it this way:

   // ship.config.js
   module.exports = {
       ...
       build: {
           ...
           minify: {
               compress: {
                   warnings: true,
                   keep_fnames: true,
               },
               mangle: {
                   keep_fnames: true,
               },
           },
       },
   };