Allow only images or all content from media feed

The changes here do not affect existing standalone functionality and defaults to the original behavior (only images are shown). This edit allows a user to turn off the images-only filter so other content types can be listed as well.

This pull request could be expanded upon to allow specific filters to be set and/or apply to the widget page / in wp-admin and not limit scope to custom / static pages. This edit also does not allow one to pull videos only (it just removes the filter when specified not using only images)

In-page usage can be dropped inline for custom or static pages like so:

<?php the_widget( 'null_instagram_widget', array(
                            'title' => 'Instagram',
                            'username' => $username,
                            'link' => 'Follow Us',
                            'wpiw',
                            'number' => 9,
                            'size' => 'thumbnail'),
                            'imagesOnly=0'
                        ); ?>