Add sqlite3_progress_handler() to API for Progress bar callback

[chaxor]

The use of SQLite.@register cxn func is awesome.

Is there any possibility that a progress bar for queries will be implemented via the progress_handler that sqlite3 offers? I would imagine it's a very similar implementation to what has been done with register or create_function.

[quinnj]

This could be really cool to hook into standard Julia progress handling packages! I could probably take a stab at something in the next few months, but if you'd like to have a go, feel free in the mean time!