added feature custom connection callback.

usage example

``` go
package main

import (
	"database/sql"
	"fmt"
	"github.com/mxk/go-sqlite/sqlite3"
)

func main() {
	sql.Register("sqlite3-hook", &sqlite3.Driver{func(conn *sqlite3.Conn) error {
		fmt.Println("connect:", conn)
		if err := conn.Exec("pragma journal_mode=WAL;" +
			"pragma read_uncommitted=true;" +
			"pragma locking_mode=NORMAL;" +
			"pragma synchronous=FULL;" +
			"pragma busy_timeout=60000;" +
			"pragma cache_spill=true;"); err != nil {
			return err
		}
		return nil
	}})
	db, err := sql.Open("sqlite3-hook", "file:db.sqlite3")
	fmt.Println(db, err)
}
```