customs output sourcemap attributes and sets sourMappingURL on banner

Changes attributes of source map.
Sets `sourceMappingURL` in the banner.
```sh
> grunt
```    
    
```js
      sourcemap_banner: {
        files: {
          'tmp/sourcemap_banner.js': ['test/fixtures/src/simple.js']
        },
        options: {
          banner: '/*\n//@ sourceMappingURL=sourcemap_banner\n*/\n',
          sourceMap: 'tmp/sourcemap_banner'
        }
      }

// output min file
/*
//@ sourceMappingURL=sourcemap_banner
*/
function longFunctionC...
```    
        
```js
      sourcemap_override: {
        files: {
          'tmp/sourcemap_override.js': ['test/fixtures/src/simple.js']
        },
        options: {
          sourceMap: 'tmp/sourcemap_override',
          sourceMapOverride: {
            file: 'simple.min.js',
            sources: ['simple.js']
          }
        }
      },

// output source map
{"version":3,"file":"simple.min.js","sources":["simple.js"],"names":"..."}
```