Allow continuing commands across linebreaks

It would help with clarity and maintenance if a command could continue across linebreaks in a scenario description. So that e.g. this line

when I run opgpcard admin --card AFAF:00001234 --admin-pin admin.pin generate --user-pin user.pin --output certfile

could instead be written somewhat like this:

when I run \
  opgpcard admin \
    --card AFAF:00001234 \
    --admin-pin admin.pin \
    generate \
      --user-pin user.pin \
      --output certfile