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.

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

Install from a fixed branch to avoid it picking a feature branch.
Use composer when adding the dep.
Use the real name in the example commands.