Bulk Insert / Ignore Option Edit

This PR adds support for the `INSERT IGNORE` option in .bulkCreate method (as mentioned in issue #1311). Using this option, if there happens to be a duplicate ID conflict, the rest of the query continues as it normally would (as opposed to throwing an error).

A usage example would be:
```
User.bulkCreate([ data ], ['username', 'age'], { ignore: true }).complete()
```

For now, this feature is only supported by the MySql + SQLite dialects... Postgres, if at all, will require some more work.