feat: add option to disable checking alt text

[7nohe]

I would like to add an option to uncheck alt text.
The reason for this is that I manage a blog where several people write posts and some do not like to write descriptions on their images.
I need this option to optimize the images for all posts.

[seesjays]

Since Eleventy Image says they're fine with alt="", I'll implement this config that way in the 2.0 release.

But, like, y'know, ask your bloggers to add alt text. Doesn't hurt anyone at all.