feat(if): Support inequality operator

Tests/StencilTests/ExpressionSpec.swift

@@ -162,7 +162,7 @@ func testExpressions() {
         }
 
         $0.it("evaluates to false with non equal lhs/rhs") {
-          print(expression)
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": "a", "rhs": "b"]))).to.beFalse()
         }
 
         $0.it("evaluates to true with nils") {
@@ -190,6 +190,18 @@ func testExpressions() {
         }
       }
 
+      $0.describe("inequality expression") {
+        let expression = try! parseExpression(components: ["lhs", "!=", "rhs"])
+
+        $0.it("evaluates to true with inequal lhs/rhs") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": "a", "rhs": "b"]))).to.beTrue()
+        }
+
+        $0.it("evaluates to false with equal lhs/rhs") {
+          try expect(expression.evaluate(context: Context(dictionary: ["lhs": "b", "rhs": "b"]))).to.beFalse()
+        }
+      }
+
       $0.describe("multiple expression") {
         let expression = try! parseExpression(components: ["one", "or", "two", "and", "not", "three"])