Fix ability to add custom patterns

Adding patterns completely replaces the patterns in fileprocessor, where it could be more useful if the pattern is appended instead.

```js
usemin: {
  html: [],
  css: [],
  options: {
    assetsDirs: [],
    patterns: {
      html: [
        [/<img[^\>]*[^\>\S]+ng-src=['"]([^"']+)["']/gm, 'Update the ng-src to reference our revved images']
      ]
    }
  }
}
```