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
         }

Sources/HummingbirdMustache/Method.swift

@@ -39,3 +39,14 @@ extension Dictionary: HBMustacheMethods {
         }
     }
 }
+
+extension Int: HBMustacheMethods {
+    func runMethod(_ name: String) -> Any? {
+        switch name {
+        case "plus1":
+            return self + 1
+        default:
+            return nil
+        }
+    }
+}

Sources/HummingbirdMustache/Sequence.swift

@@ -15,6 +15,7 @@ extension Sequence {
             string += template.render(currentObject, context: context)
             currentObject = object
             context.first = false
+            context.index += 1
         }
 
         context.last = true