add toggleable option to activate truely view-once-and-destroy mode

[whysthatso]

Is your feature request related to a problem? Please describe.
when reloading the same tab, the information is still present, related to the payload value in the sessionstorage. i understand this is a safety measurement as discussed in #40

Describe the solution you'd like
i'd like to have a configurable 'fragile' option which activates message destruction upon viewing, without any safety net. default can be 'off', but then a notification should show that the message is deleted remotely but will only be deleted locally once the browser is closed / the browser session is ended (should be very easily understandable by an end user)

Additional context
See discussion #40

[rendler-denis]

hi, @whysthatso

I finally managed to get around to adding this. :)
I've added a cache: true or false option under the static-site > envjs config. By default it is true, but if you want to disable it just put false in your config.

Let me know if there are any bugs.