feat: Allow subclassing templates (#79)

Tests/StencilTests/EnvironmentSpec.swift

@@ -25,11 +25,17 @@ func testEnvironment() {
       let result = try environment.renderTemplate(name: "example.html")
       try expect(result) == "Hello World!"
     }
+
+    $0.it("allows you to provide a custom template class") {
+      let environment = Environment(loader: ExampleLoader(), templateClass: CustomTemplate.self)
+      let result = try environment.renderTemplate(string: "Hello World")
+
+      try expect(result) == "here"
+    }
   }
 }
 
 
-
 fileprivate class ExampleLoader: Loader {
   func loadTemplate(name: String, environment: Environment) throws -> Template {
     if name == "example.html" {
@@ -39,3 +45,10 @@ fileprivate class ExampleLoader: Loader {
     throw TemplateDoesNotExist(templateNames: [name], loader: self)
   }
 }
+
+
+class CustomTemplate: Template {
+  override func render(_ dictionary: [String: Any]? = nil) throws -> String {
+    return "here"
+  }
+}