Fix ability to add custom patterns

Using yeoman angular generator, I want to be able to add custom patterns but currently it replaces the default ones. This is how my gruntfile's usemin entry looks like:
```
usemin: {
      html: ['<%= yeoman.dist %>/{,*/}*.html'],
      css: ['<%= yeoman.dist %>/styles/{,*/}*.css'],
      options: {
        assetsDirs: ['<%= yeoman.dist %>','<%= yeoman.dist %>/images'],
        patterns: {
          html: [
            [/<img[^\>]*[^\>\S]+ng-src=['"]([^"']+)["']/gm, 'Update the ng-src to reference our revved images']
          ]
        }
      }
    }
```