Specify if search should be case-insensitivity

I think it should be a good feature to add case-insensitivity.

So that would be this in MySql:
```sql
LOWER(posts.title) LIKE (?)
```

or this in PostgeSQL:
```sql
posts.title ILIKE (?)
```

So, maybe it could be:
```ruby
Post.where.ilike(title: "%rails%")
Post.where.not_ilike(title: "%rails%")
```