Add an option to delete a symlink

Hi,

I've added a small feature to this grunt plugin which allows you to set an option called `delete`. If set to true, it will not create a symlink pointed by `dest` but delete it (if it exists).

I've also added an additional check in the regular symlink add code path to validate that the overwritten path is indeed a symlink (preventing the deletion of existing files/folders).

Note that the change looks quite big simply because I ended up indenting most of it because of the initial check to options.delete.

Let me know what you guys think and if there's anything that could be improved here.
Thanks