Add index filter

Sources/HummingbirdMustache/Context.swift

@@ -2,10 +2,12 @@
 struct HBMustacheContext: HBMustacheMethods {
     var first: Bool
     var last: Bool
+    var index: Int
     
     init(first: Bool = false, last: Bool = false) {
         self.first = first
         self.last = last
+        self.index = 0
     }
 
     func runMethod(_ name: String) -> Any? {
@@ -14,6 +16,8 @@ struct HBMustacheContext: HBMustacheMethods {
             return self.first
         case "last":
             return self.last
+        case "index":
+            return self.index
         default:
             return nil
         }