Add configuration option to ignore files (prevent them from being minified)

Some JS libraries break when minimized (see for example, https://github.com/brunch/uglify-js-brunch/issues/12), so there should be an easy (or easier) way to pass them through with no minimization. Methods such as copying to assets are breaking the use of bower, and the normal brunch flow.

This is not the most elegant solution, but better solutions might be better done at brunch, and not the specific plugin.

Suggested way of usage in config.coffee:
```coffeescript
  files:
    javascripts:
      joinTo:
        'js/non_minimized.js': /^bower_components\/rickshaw/
        'js/app.js': /^app/
        'js/vendor.js': /^(bower_components\/(?!rickshaw)|vendor)/
#...
  plugins:
    uglify:
      ignored: /non_minimized\.js/
#...

```