Allow current node as an expression

This now passes the latest compliance tests.

Author: jamesls

jmespath.js

@@ -528,6 +528,10 @@
           return {type: "ValueProjection", children: [left, right]};
       },
 
+      nudCurrent: function() {
+          return {type: "Current"};
+      },
+
       nudLbracket: function() {
           if (this.lookahead(0) === "Number" || this.lookahead(0) === "Colon") {
               var right = this.parseIndexExpression();

test/compliance/current.json

@@ -0,0 +1,25 @@
+[
+    {
+        "given": {
+            "foo": [{"name": "a"}, {"name": "b"}],
+            "bar": {"baz": "qux"}
+        },
+        "cases": [
+            {
+                "expression": "@",
+                "result": {
+                    "foo": [{"name": "a"}, {"name": "b"}],
+                    "bar": {"baz": "qux"}
+                }
+            },
+            {
+                "expression": "@.bar",
+                "result": {"baz": "qux"}
+            },
+            {
+                "expression": "@.foo[0]",
+                "result": {"name": "a"}
+            }
+        ]
+    }
+]

test/compliance/syntax.json

@@ -95,14 +95,6 @@
       {
         "expression": "a][",
         "error": "syntax"
-      },
-      {
-        "expression": "@",
-        "error": "syntax"
-      },
-      {
-        "expression": "@.foo",
-        "error": "syntax"
       }
     ]
   },