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.
```
usemin: {
      html: [],
      css: [],
      options: {
        assetsDirs: [],
        patterns: {
          html: [
            [/<img[^\>]*[^\>\S]+ng-src=['"]([^"']+)["']/gm, 'Update the ng-src to reference our revved images']
          ]
        }
      }
    },
```