Allow copying command only from shell code blocks #396

On my blog I show code blocks with content copied my terminal where I like to show

• the shell prompt, e.g. [thib@bigbox ~]$, so readers can know who entered the command and where
• the command itself of course, e.g. kopia repository create. That command can be on several lines (see Support line breaks with \ in shell code blocks #344)
• the output of the command, so readers know what to expect on their machine

It would be very useful if the Copy button could only copy the command and not the whole code block :)