Allow customizing (or disabling) timestamp format

[nhandler]

Allow the user to specify a strftime format (with support for an empty string) for controlling the format of the timestamp added to all images.

[matfra]

Is it possible with rapsistill? Ideally I would like to display the exif date (or file creation) using JavaScript. But then I need to add this logic to the timelapse script as well and
I always wondered how would that play with the HW acceleration (raspberry pi zero doesn't have a lot of resources)