and error out on internal ones

The old name "secret_token" implied a bit that it is required for something,
but it was just an example. This new name should make it more clear.

So the bundle doesn't fail "composer install" initially.
The config is just an example, so it's not very important.

This adds one config entry and injects the value into a service when the bundle gets loaded.

See the README changes for how to set the config in the symfony app.

Fixes #2