add contex to include

Sources/Include.swift

@@ -3,19 +3,21 @@ import PathKit
 
 class IncludeNode : NodeType {
   let templateName: Variable
+  let includeContext: String?
 
   class func parse(_ parser: TokenParser, token: Token) throws -> NodeType {
     let bits = token.components()
 
-    guard bits.count == 2 else {
-      throw TemplateSyntaxError("'include' tag takes one argument, the template file to be included")
+    guard bits.count == 2 || (bits.count == 4 && bits[2] == "using") else {
+      throw TemplateSyntaxError("'include' tag requires one argument, the template file to be included. Another optional argument can be used to specify the context that will be passed to the included file, using the format \"using myContext\"")
     }
 
-    return IncludeNode(templateName: Variable(bits[1]))
+    return IncludeNode(templateName: Variable(bits[1]), includeContext: bits.count == 4 ? bits[3] : nil)
   }
 
-  init(templateName: Variable) {
+  init(templateName: Variable, includeContext: String? = nil) {
     self.templateName = templateName
+    self.includeContext = includeContext
   }
 
   func render(_ context: Context) throws -> String {
@@ -25,7 +27,8 @@ class IncludeNode : NodeType {
 
     let template = try context.environment.loadTemplate(name: templateName)
 
-    return try context.push {
+    let subContext = includeContext.flatMap{ context[$0] as? [String: Any] }
+    return try context.push(dictionary: subContext) {
       return try template.render(context)
     }
   }