Add ability to update imported content

I like the module, however I thought some people might find it useful to be able to have imported nodes continue to update so that like/comment counts may be kept up to date.

Attached is a patch that allows nodes to be 'updated' while the import function runs. This would require slight refactoring of how each sub-module calls parent::prepareRow. I have only refactored Facebook and Instagram in this patch.

This would probably be best done as an option on each importer instance.