Document Ctrl-C and Ctrl-Z behavior and prompt themes

[Kaligule]

It would be cool if we could have some more documentation in the help text.

• Explain how to use Ctrl-C and Ctrl-Z in a presentation (this is self explanatory for CLI veterans, but many users might not be so experienced. Ctrl-C and Crtl-Z are really a great part of how to use the programm in a presentation)
• Explain how <prompt_themes> work: How can I create my own? Where can I find premade ones?

[sloria]

I agree--these would be nice to document. PRs welcome =)