enable to specify MySQL extra parameters, especially to support non-ascii characters

I added `:extra` option into `korma.db/mysql` function.

To support non-ascii characters in MySQL, we have to append some query parameters JDBC URI, such as `?useUnicode=true&characterEncoding=utf8`.

Using this PR, you can specify query parameters as hash-map, for example,

```clojure
(mysql {:db "mydb" :user "root" :extra {"useUnicode" true "characterEncoding" "utf8"}})
```

Thanks.