This doesn't really solve a pressing problem, because I guess it's unlikely that
users add spaces at the beginning or end of what they type into a prompt; but it
could happen, and in this case we almost always want to strip it. Just adding
this here for completeness while I was working on this code.
The only exception is the input prompt of custom commands, because who knows
what users want to use that input for in their custom command.
This might seem controversial; in many cases the client code gets longer,
because it needs an extra line for an explicit `return nil`. I still prefer
this, because it makes it clearer which calls can return errors.
