Add option to clear profiles before import

Adds option to delete all profiles before importing new ones. Unchecked by default.

Checks first that there are legitimate profiles to import, so it doesn't leave user with a blank profile.

Have tested it importing my own profile from Firefox, exporting and re-importing. Also exporting and re-importing some test profiles made from clean Chrome.

![delete-profile-before-import](https://f.cloud.github.com/assets/1316058/39735/f8bd90a8-5540-11e2-9054-102820a66f8c.png)