claat: support user-input in command-line snippets

[simos]

The command-line snippet can be enhanced in order to show which is the user-input text among the text output of the commands.

Existing codelabs are using different conventions in showing command-line commands. Some are omitting the shell prompt altogether, others do not show any output from the commands which can be confusing to the users.

For a codelabs author, they should just make bold the parts of the command-line input that is actual user-input. Then claat should show this text in a distinctive way to denote user-input.

See screenshot of commands with user-input in bold

[x1ddos]

Yeah, many codelabs are not uniform. The disadvantage of this is that a hypothetical click-to-copy button would be useless. We were trying to encourage authors to provide snippets without the a shell prompt.

Not sure about pros and cons though.