Add :cache option to the filepicker_image_url helper

From the api documentation at https://developers.filepicker.io/docs/web/#fpurl-contents

**Cache Settings**
Using the `cache=true` query parameter, we'll set the file not to expiry. The `cache=false` parameter will ask the browser not the cache the file.

```
>>> curl -I https://www.filepicker.io/api/file/hFHUCB3iTxyMzseuWOgG?cache=true
HTTP/1.1 200 OK
...
Cache-Control: public, max-age=315360000, no-transform
...
>>> curl -I https://www.filepicker.io/api/file/hFHUCB3iTxyMzseuWOgG?cache=false
HTTP/1.1 200 OK
...
Cache-Control: no-cache
...
```