Fixed using spaces in filter expressions and variables lists (#178)

* fixed using spaces in filter expression

* fixed breaking variables lists and filters by spaces

* simplified smartJoin

* avoid force unwrap

Sources/ForTag.swift

@@ -24,7 +24,7 @@ class ForNode : NodeType {
     let loopVariables = components[1].characters
       .split(separator: ",")
       .map(String.init)
-      .map { $0.trimmingCharacters(in: CharacterSet.whitespaces) }
+      .map { $0.trim(character: " ") }
 
     var emptyNodes = [NodeType]()