Always have a default configuration available
Always have a default configuration available

The previous implementation depended on `Clearance.configure` being
called in an initializer, otherwise `Clearance.configuration` would be
nil. This change makes Clearance work without having to call
`Clearance.configure` first. The initializer is now only needed if you
want to override a default.

benlivermore/clearance,janusnic/clearance,zernie/clearance,benlivermore/clearance,naveed-ahmad/clearance,thoughtbot/clearance,thoughtbot/clearance,thoughtbot/clearance,sferik/clearance,pedrosmmoreira/clearance,naveed-ahmad/clearance,benlivermore/clearance,zernie/clearance,pedrosmmoreira/clearance,naveed-ahmad/clearance,janusnic/clearance,zernie/clearance,janusnic/clearance,pedrosmmoreira/clearance