Add function to register wal hook

[gfanto]

I have the use case of adding a custom wal hook.
This PR adds support for registering a WAL hook on a connection via a new method:

func (c *SQLiteConn) RegisterWalHook(callback func(dbName string, nPages int) int)

[mattn]

There's a typo in callback.go: hadlePtr should be handlePtr.