Add `fallback` option specs
Refactor allow `fallback_authentication_handler` to receive options
Refactor rename `FallbackAuthenticationHandler`...

... to `DeviseAuthenticationHandler`, so other other authentication
handlers can be used as fallback.
Add `FailedAuthenticationHandler`

It implements the 'authentication handler' interface and is intended
to be used as a fallback when token authentication fails.
Add option: `fallback: :exception`, use `FailedAuthenticationHandler`

This is the first step to add support for exception throwing on
authentication failure.
Minor fix indentation

This commit was left distinct of the previous one to ensure
the previous diff readability.