add PROMPT_COMMAND to the front of prompt-command

If PROMPT_COMMAND was already set, prior to entering the prompt-command
component, this information would be lost. To correct this, add the
current $PROMPT_COMMAND to a key of prompt-command's array, if that
$PROMPT_COMMAND is non-empty.

This will *not* play nicely with being re-loaded repeatedly in all but
the most trivial cases.