Prompt user to replace existing bookmark.

When a user tries to add a bookmark by a name that already exists,
Bashmarks gives them the option to replace their existing bookmark.
![Screen Shot 2013-03-04 at 8 46 00 PM](https://f.cloud.github.com/assets/431529/219952/a7162334-854f-11e2-91cb-7cba68fbe7d2.png)