Allow conditions in variable node (#243)

* use condition in variable node

* added support for else expression

* addressing code review comments

Sources/Parser.swift

@@ -40,8 +40,7 @@ public class TokenParser {
       case .text(let text, _):
         nodes.append(TextNode(text: text))
       case .variable:
-        let filter = try compileResolvable(token.contents, containedIn: token)
-        nodes.append(VariableNode(variable: filter, token: token))
+        try nodes.append(VariableNode.parse(self, token: token))
       case .block:
         if let parse_until = parse_until , parse_until(self, token) {
           prependToken(token)