add support for insert, update and delete queries

Hi,

I've added basic support for insert, update and delete queries.
Here are some examples:

```clojure
(format {:insert-into :foo
         :values {:id 1 :name "bob"}})
;= ["INSERT INTO foo (name, id) VALUES (?, 1)" "bob"]

(format {:insert-into :foo
         :values [1 "bob"]})
;= ["INSERT INTO foo VALUES (1, ?)" "bob"]

(format {:update :foo
         :set {:name "bob" :description "hi"}
         :where [:= :id 123]})
;= ["UPDATE foo SET name = ?, description = ? WHERE id = 123" "bob" "hi"]

(format {:delete-from :foo
         :where [:= :id 123]})
;= ["DELETE FROM foo WHERE id = 123"]
```

I haven't updated README.md. I couldn't decide where is the appropriate location to put it.