Add a keyword node to use "*" and "not" in Squeel.

Hi @ernie,

In #17, #182, people mentioned problems about using some keywords in SQL, it's hard to implement in Ruby, @unrealhoang said if it had a possibility to use a trailing character before these keywords.

I think things should like :

```ruby
# _* is invalid...
Person.select{articles._star}.joins{articles}
Person.select{_star}
Person.where{(id == 1) | _not(exists(Article.where{person_id == people.id}))}
```

But I'm not sure that things I changed was proper. So it's very glad to hear your suggestions :smile: