Implement if and ifnot tags

Closes #3

StencilTests/NodeTests.swift

@@ -86,3 +86,71 @@ class ForNodeTests: NodeTests {
         XCTAssertEqual(result.0!, "123")
     }
 }
+
+class IfNodeTests: NodeTests {
+
+    // MARK: Parsing
+
+    func testParseIf() {
+        let tokens = [
+            Token.Block(value: "if value"),
+                Token.Text(value: "true"),
+            Token.Block(value: "else"),
+                Token.Text(value: "false"),
+            Token.Block(value: "endif")
+        ]
+
+        let parser = TokenParser(tokens: tokens)
+        let nodes = parser.parse()
+        let node = nodes.first! as IfNode
+        let trueNode = node.trueNodes.first! as TextNode
+        let falseNode = node.falseNodes.first! as TextNode
+
+        XCTAssertEqual(nodes.count, 1)
+        XCTAssertEqual(node.variable.variable, "value")
+        XCTAssertEqual(node.trueNodes.count, 1)
+        XCTAssertEqual(trueNode.text, "true")
+        XCTAssertEqual(node.falseNodes.count, 1)
+        XCTAssertEqual(falseNode.text, "false")
+    }
+
+    func testParseIfNot() {
+        let tokens = [
+            Token.Block(value: "ifnot value"),
+                Token.Text(value: "false"),
+            Token.Block(value: "else"),
+                Token.Text(value: "true"),
+            Token.Block(value: "endif")
+        ]
+
+        let parser = TokenParser(tokens: tokens)
+        let nodes = parser.parse()
+        let node = nodes.first! as IfNode
+        let trueNode = node.trueNodes.first! as TextNode
+        let falseNode = node.falseNodes.first! as TextNode
+
+        XCTAssertEqual(nodes.count, 1)
+        XCTAssertEqual(node.variable.variable, "value")
+        XCTAssertEqual(node.trueNodes.count, 1)
+        XCTAssertEqual(trueNode.text, "true")
+        XCTAssertEqual(node.falseNodes.count, 1)
+        XCTAssertEqual(falseNode.text, "false")
+    }
+
+    // MARK: Rendering
+
+    func testIfNodeRenderTruth() {
+        let node = IfNode(variable: "items", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
+        let result = node.render(context)
+
+        XCTAssertEqual(result.0!, "true")
+    }
+
+    func testIfNodeRenderFalse() {
+        let node = IfNode(variable: "unknown", trueNodes: [TextNode(text: "true")], falseNodes: [TextNode(text: "false")])
+        let result = node.render(context)
+
+        XCTAssertEqual(result.0!, "false")
+    }
+
+}