Fix ability to add custom patterns

I am using generator-yeoman and there's already a pattern option but looks like it's replacing the patterns list instead of adding in custom ones. This mod works for my current needs given the config below:
```
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']
      ]
    }
  }
}
```