Add support for Lambdas

Sources/HummingbirdMustache/Lambda.swift

@@ -0,0 +1,2 @@
+
+public typealias HBMustacheLambda = (Any, HBMustacheTemplate, HBMustacheLibrary?) -> String

Sources/HummingbirdMustache/Template+Render.swift

@@ -41,6 +41,8 @@ extension HBMustacheTemplate {
             return array.renderSection(with: template, library: library)
         case let bool as Bool:
             return bool ? template.render(parent, library: library) : ""
+        case let lambda as HBMustacheLambda:
+            return lambda(parent, template, library)
         case .some(let value):
             return template.render(value, library: library)
         case .none:

Tests/HummingbirdMustacheTests/TemplateRendererTests.swift

@@ -153,6 +153,19 @@ final class TemplateRendererTests: XCTestCase {
             """)
     }
     
+    func testMustacheManualExample5() throws {
+        let template = try HBMustacheTemplate(string: """
+            {{#wrapped}}{{name}} is awesome.{{/wrapped}}
+            """)
+        func wrapped(object: Any, template: HBMustacheTemplate, library: HBMustacheLibrary?) -> String {
+            return "<b>\(template.render(object, library: library))</b>"
+        }
+        let object: [String: Any] = ["name": "Willy", "wrapped": wrapped]
+        XCTAssertEqual(template.render(object), """
+            <b>Willy is awesome.</b>
+            """)
+    }
+    
     func testMustacheManualExample6() throws {
         let template = try HBMustacheTemplate(string: """
             {{#person?}}