Make ag search respect ignored patterns.

@bbatsov

Make `projectile-ag` ignore patterns from `.projectile` when performing search. The only drawback that I'm aware of is that it's not possible to specify to ignore files only in a direct subtree of the root of the project. E.g. given the following directory structure:

```
root
|- foo
   |- test.txt
|- bar
|- baz
   |- foo
      |- test.txt
```

There's no `ag --ignore` syntax that would allow you to ignore just the top level `foo/test.txt` file. It will always exclude `baz/foo/test.txt` too.

I've tested this by playing w/ `.projectile` file and confirmed that it correctly ignores the patterns specified.
I have no idea why you're using the prefix arg here to determine how to obtain the project root. Pretty sure you should always use `projectile-project-root`.