issue #9: add optional prefix to remove from absolute path

Given the additional storage provider specific option removePrefix:

```
ImageSchema.plugin(attachments, {
    directory: config.images,
    storage : {
        providerName: 'localfs',
        options : {
          removePrefix: config.public  
        }        
    },
...

We remove that prefix. This lets us have a directly usable relative path-part of the URL in our frontend application.