cli: Add "stoken --next" convenience option

This adds (interval) seconds so that the CLI displays the next tokencode.
For a 60-second token this is equivalent to "--use-time +60".

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
man: Add --next option; tweak formatting

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>