Add `prefix` and `suffix`

# Prefix and Suffix options

- Adds option.prefix and option.suffix to prepend and append text to an attribute
- Updates tests with new options
- Updates README with new options

These options are particularly helpful if you're trying to deploy to GitHub Pages and need to add a path to all links relative to `/`

## Example using Yeoman:

```js
    dom_munger:      {
      update_links_and_css: {
        options: {
          prefix: {
            selector:  '[href^="/"]',
            attribute: 'href',
            value:     '/project-name'
          }
        },
        src:     '<%= yeoman.dist %>/**/*.html'
      },
      update_js_and_images: {
        options: {
          prefix: {
            selector:  '[src^="/"]',
            attribute: 'src',
            value:     '/project-name'
          }
        },
        src:     '<%= yeoman.dist %>/**/*.html'
      }
    }
```