[sebascarra]

Allow slide duration in seconds

• Change the slide duration setting to seconds, with a 900-second (15-minute) maximum.
• Persist the selected seconds value directly and update the README range.
• Add a config persistence assertion for seconds.

[sebascarra]

@micw hey, your project is awesome, but I was wondering if you'd be happy to allow us to set the slide duration in seconds instead of minutes. I'm fine with keeping increments of 5 seconds if you think it's a better idea. If so let me know and I'll change it. Or even if you prefer a more limited amount of settings, I'm fine with it too. Just let us know how we could move forward with this as I've seen a few folks request the feature already. Thanks!