Adds a parameter to Query::insert() to allow INSERT IGNOREs

adds a second parameter to Query::insert() to allow ignores. e.g:
```php
$data = array(
  array('email' => 'example@gmail.com'),
  array('email' => 'example@yahoo.com')
);
DB::table('users')->insert($data, true);
```
will yield:
```sql
INSERT IGNORE INTO `posts` (`email`) VALUES (?), (?)
Bindings: array (
  0 => 'example@gmail.com',
  1 => 'example@yahoo.com'
)
```
Without the 2nd parameter, insert behavior remains intact.