Support passing command prefix to cmd

[nlgranger]

I think it would be neat if cmd could simulate typing the beginning of the command.
For instance:

pe "htop"
# Copy-paste some pid
cmd "kill "
# wait for enter key like p, pe etc.
# simulates typing kill
# presenter pastes the pid and presses return to execute

Also, if you forgot you were on the cmd step, you don't accidentally end up executing an empty command while you were trying to trigger the next step. Instead you trigger something which has a visual feedback. I think this is better in terms of UX.