Feature request: exit early if `--crawl-no-parent` and friends are specified without `--crawl-links`

[brasswood]

I was running single-file with --crawl-no-parent and it wasn't crawling. I was scratching my head until I looked at the source and saw that you have to specify --crawl-links in order to do any crawling, then --crawl-no-parent modifies the way the crawling is done.

It would help with the user interface if the command exited with an error message if --crawl-no-parent and friends are specified without --crawl-links. I'm sure there are more options like this that depend on other options being enabled.

[brasswood]

Another option (since some people want empathic CLIs) could be to have --crawl-no-parent imply --crawl-links.