Introduce provider registry

src/multi_auth/engine.cr

@@ -1,14 +1,6 @@
 class MultiAuth::Engine
   def initialize(provider : String, redirect_uri : String)
-    provider_class = case provider
-                     when "google"   then Provider::Google
-                     when "github"   then Provider::Github
-                     when "facebook" then Provider::Facebook
-                     when "vk"       then Provider::Vk
-                     when "twitter"  then Provider::Twitter
-                     else
-                       raise "Provider #{provider} not implemented"
-                     end
+    provider_class = MultiAuth::Providers.get(provider)
 
     key, secret = MultiAuth.configuration[provider]
     @provider = provider_class.new(redirect_uri, key, secret)

src/multi_auth/providers.cr

@@ -0,0 +1,11 @@
+class MultiAuth::Providers
+  @@registry = {} of String => MultiAuth::Provider.class
+
+  def self.register(name : String, klass : MultiAuth::Provider.class)
+    @@registry[name] = klass
+  end
+
+  def self.get(name : String)
+    @@registry[name]
+  end
+end

src/multi_auth/providers/facebook.cr

@@ -80,3 +80,5 @@ class MultiAuth::Provider::Facebook < MultiAuth::Provider
     )
   end
 end
+
+MultiAuth::Providers.register("facebook", MultiAuth::Provider::Facebook)

src/multi_auth/providers/github.cr

@@ -72,3 +72,5 @@ class MultiAuth::Provider::Github < MultiAuth::Provider
     )
   end
 end
+
+MultiAuth::Providers.register("github", MultiAuth::Provider::Github)

src/multi_auth/providers/google.cr

@@ -115,3 +115,5 @@ class MultiAuth::Provider::Google < MultiAuth::Provider
     user
   end
 end
+
+MultiAuth::Providers.register("google", MultiAuth::Provider::Google)

src/multi_auth/providers/twitter.cr

@@ -65,3 +65,5 @@ class MultiAuth::Provider::Twitter < MultiAuth::Provider
     @consumer ||= OAuth::Consumer.new("api.twitter.com", key, secret)
   end
 end
+
+MultiAuth::Providers.register("twitter", MultiAuth::Provider::Twitter)

src/multi_auth/providers/vk.cr

@@ -104,3 +104,5 @@ class MultiAuth::Provider::Vk < MultiAuth::Provider
     )
   end
 end
+
+MultiAuth::Providers.register("vk", MultiAuth::Provider::Vk)