Add ability to register template engine outside the library

src/kilt.cr

@@ -2,17 +2,55 @@ require "ecr/macros"
 require "./kilt/*"
 
 module Kilt
+  # macro only constants
+  TEMPLATES = [] of Int32
+  OVERRIDES = [] of Int32
+
+  macro register_template(extension, embed_macro)
+    {% ::Kilt::TEMPLATES << {extension, embed_macro} %}
+    ::Kilt._override_embed
+  end
 
   macro embed(filename, io_name = "__kilt_io__")
-    {% if filename.ends_with?(".ecr") %}
-      embed_ecr({{filename}}, {{io_name}})
-    {% elsif filename.ends_with?(".slang") %}
-      embed_slang({{filename}}, {{io_name}})
-    {% else %}
-      raise Kilt::Exception.new("Unsupported template type \"" + {{filename.split(".").last}} + "\"")
-    {% end %}
+    {{::Kilt::OVERRIDES.last.id}}.embed({{filename}}, {{io_name}})
   end
 
+  macro _override_embed
+    {% override_name = "::Kilt::Override#{OVERRIDES.size}" %}
+
+    module {{override_name.id}}
+      macro embed(filename, io_name)
+        {% start = true %}
+
+        {% for template in ::Kilt::TEMPLATES %}
+          {% extension = template[0] %}
+          {% embed_macro = template[1] %}
+
+          {% if start == true %}
+            {% start = false %}
+
+            \{% if filename.ends_with?({{extension}}) %}
+              {{embed_macro.id}}(\{{filename}}, \{{io_name}})
+
+          {% else %}
+
+            \{% elsif filename.ends_with?({{extension}}) %}
+              {{embed_macro.id}}(\{{filename}}, \{{io_name}})
+
+          {% end %}
+        {% end %}
+
+            \{% else %}
+              raise Kilt::Exception.new("Unsupported template type \"" + \{{filename.split(".").last}} + "\"")
+            \{% {{:end.id}} %}
+      end
+    end
+
+    {% ::Kilt::OVERRIDES << override_name %}
+  end
+
+  register_template(".ecr", embed_ecr)
+
   macro file(filename, io_name = "__kilt_io__")
     def to_s({{io_name.id}})
       Kilt.embed({{filename}}, {{io_name}})