Add user setting to determine whether or not to sideload images

[jmichaelward]

Sideloading images into the media library from BoardGameGeek is an intensive task, particularly if game collections are especially large. It may be advantageous to allow users to disable this feature so they can manually add their own images (if desired), or simply not have any images at all.

Related: the BoardGameGeek API route for collections includes an optional brief parameter which allows one to retrieve a shortened response about a collection, which could be used for creating Game posts and nothing more.

[jmichaelward]

Related to #3, though I do think there's a distinction between running images separately and having an option to not run images at all.

I'm going to focus on this task for the next milestone (providing a flag to ignore images), then figure out how to make images run second when the flag is not present.

[jmichaelward]

As of this week, the wp bgc update command has been refactored to exclude images by default, but includes a new optional --with-images option.

In addition to this, I need to add a check-box to the Dashboard UI to indicate that the update should include image processing. I also opened #34, which intends to resolve the issue with bulk image loading, and instead aims to perform some per-game image processing, loading the BGG image if no featured image is set, but retrieving that image and sideloading it during a request.