Lints!

[siefkenj]

There are various lints that would be helpful if the CLI reported. It is reasonable that these are implemented in the CLI so that someone who has a github action to compile a pretext book could set a flag like "no failing lints" as part of the CI tests.

• An <image> @sourcecannot be an external URL, likehttp://example.com/image.jpg`. A warning should be issued if someone types this.