Add set_update_hook/2

[greg-rychlewski]

Closes #221

I took a stab at this based on how the erlang sqlite library is handling it.

The implementation is pretty straight forward. The user supplies a connection and a pid and we create a callback that sends a message to the pid of the form {:insert | :update | :delete, db_name, table, row_id} (that's all the information SQLite exposes about the change. For example, you could create a GenServer to listen to the notifications.

Right now, there can only be one pid per database connection. But I think it can probably be extended later to work with a list of pids.

[greg-rychlewski]

This is exposed on the driver level but I think if someone really wanted to use it with Ecto they could use the after_connect configuration option and supply {Exqlite.Sqlite3, :set_update_hook, [genserver_name]}.

I believe this will work and also be called on reconnects but haven't tested. That's why I didn't advertise it in the docs yet.

[warmwaffles]

I wonder if we could capture the App.Repo atom and store it with the connection and pass that in the callback instead of the raw db name. This would probably change how the sqlite database is opened.

[warmwaffles]

This is good. I think this is probably the simplest way to implement this without having to do some changes on connection construction.

[greg-rychlewski]

Oh I was looking into whether Ecto passes the repo into the connect function. Would you be interested in a separate PR for that if it works?

[warmwaffles]

Yea I think that would be fine. I have a feeling it may be a breaking api change. Not entirely sure.