Unify printing

[mmastrac]

We have 6 different ways to print in the CLI. println/log/color_print support {} style parameters. print_markdown has its own placeholders and print is just strings.

• [ ] {e,}println
• [x] ~echo!~ (now msg!)
• [ ] log::*
• [ ] print_markdown!
• [ ] print:: (partially complete error -> error! and warn -> warn!)
• color_print/cformat

Not currently counting the prompt and question APIs which also appear to be multiple...