Add support for first/last functions

Sources/HummingbirdMustache/Context.swift

@@ -1,10 +1,22 @@
 
-struct HBMustacheContext {
-    let first: Bool
-    let last: Bool
+struct HBMustacheContext: HBMustacheMethods {
+    var first: Bool
+    var last: Bool
     
     init(first: Bool = false, last: Bool = false) {
         self.first = first
         self.last = last
     }
+
+    func runMethod(_ name: String) -> Any? {
+        switch name {
+        case "first":
+            return self.first
+        case "last":
+            return self.last
+        default:
+            return nil
+        }
+    }
+
 }