It can take quite a while to record a demo. This patch adds a `--fast`
flag for reducing delays to a tenth of the usual. You can play the
recording with `asciinema play -s 0.1` to get close-to-normal speed
(except that command delays will be slower). That way you can adjust
timings with shorter round-trips.
I find it frustrating to wait for the typing, but I need much more
time to think between commands (and users who are new to the tool
surely need even more time).
Most commands are non-interactive, so this reduces duplication. We can
add another function for running a command and not waiting for a
prompt when we need it.
