feat: Add DictionaryLoader to load templates from dictionary

Closes #97

Tests/StencilTests/LoaderSpec.swift

@@ -29,4 +29,27 @@ func testTemplateLoader() {
       try expect(try environment.loadTemplate(name: "../LoaderSpec.swift")).toThrow()
     }
   }
+
+  describe("DictionaryLoader") {
+    let loader = DictionaryLoader(templates: [
+        "index.html": "Hello World"
+    ])
+    let environment = Environment(loader: loader)
+
+    $0.it("errors when a template cannot be found") {
+      try expect(try environment.loadTemplate(name: "unknown.html")).toThrow()
+    }
+
+    $0.it("errors when an array of templates cannot be found") {
+      try expect(try environment.loadTemplate(names: ["unknown.html", "unknown2.html"])).toThrow()
+    }
+
+    $0.it("can load a template from a known templates") {
+      _ = try environment.loadTemplate(name: "index.html")
+    }
+
+    $0.it("can load a known template from a collection of templates") {
+      _ = try environment.loadTemplate(names: ["unknown.html", "index.html"])
+    }
+  }
 }