treat all non-zero exit status as error

[tomeichlersmith]

This is an alternative solution to #19 and resolves #18 .

Instead of introducing new (perhaps clunky) syntax, cmdrun just treats all non-zero exit codes as errors and injects a markdown message into the book describing this to the user. Simply appending || true (like in the diff example) can inform cmdrun that your command is intentional and should be allowed to produce output for the book.