Add option to crash on start when required vars are not set

[SpencerKaiser]

Pre-requisites:

• Yes, I looked through both open and closed issues looking for what I needed

Feature Description

simple-env should include an option to fail to start if a required variable is not set; this would prevent the app from starting but failing in prod because something required was missing at runtime.

For example, a Slack token necessary to respond to an event may be unset when the app starts, the app could start successfully, blue green could swap the apps successfully, and the app would then be in a broken state.

[johnkahn]

I think this should actually be the default and we should have an option to disable it